There is no doubt that what is being claimed to have been done in this case is the alteration of a number on a plate. In order to substantiate that this claim is plausible and possible, experiments must be conducted in order to prove that this is practical and can actually be physically done.

An experienced engraver a Mr. David Tallett was commissioned to engrave the figure '73' within two diamonds and then alter the right hand figure '3' into a figure '7', in order to create the number '77'.

The images below, which were taken at stages as the work was being done, clearly show the ease by which such a task could have been carried out by the engravers at Perkins Bacon, who undoubtedly had more experience in engraving stamp plates than Mr. Tallett.

In this case the '7' was created in part from the existing '3' and some from the recessed metal.

In the event of a number that needed creating from a recessed area where there is no raised metal to 'mould' into the desired figure, then I am advised by Mr. Tallett that a small recess can be dug out and some soft filler metal tapped in on which the new number can be engraved. I am assured that this is a very simple process for an experienced engraver to carry out and one that can be done in the smallest of areas. To illustrate this possibility a sample was produced by inserting a gold plug into a predefined area and engraving it with the figure '7'. This is also illustrated below.

The possibility of a recess being created and a metal plug applied onto which the number '7' was engraved can therefore not be dismissed and this likely possibility is also in need of investigation. A new metal introduced to the plate may bring with it the contaminants that where found using Raman spectroscopy and EDXRF

There is absolutely no doubt that the engravers of the time were more than capable of fixing a plethora of problems that could have originated from plate wear and which would have needed repair in one way or another. It will be wrong for us to judge what can and can not be done by these experienced engravers bearing in mind that most of us have little to no knowledge of engraving or of the 'tricks of the engraving trade' that would have been used.

The trace elements found in the right hand '7'.

The engraved area on the plate.

It is quite clear in this case that the number '77' was re-engraved from a number '73' on the plate. For this to be done, the diamond area in which the number resides must be cleared from ink residues that would have been left behind from the previous printing process in order to free the existing number.

While we can not know for certain how this cleaning process may have been carried out, it is possible to visualise a suitable 'cleaning' fluid being used for this purpose, the formula for which may well have been a 'tarde secret'.

While my research to date has not uncovered any formula for such a 'fluid', the possibility of one being made from compounds indicated by the present trace elements can not be ruled out. We know that trace chromium, phosphorus and barium were found.

One possible fluid therefore may perhaps contain one, two of these compounds; chromic acid, phosphoric acid. While perhaps we may never know the exact formula for this fluid, we do have the challenge of this research if we wish to take it up.

1- Trace Chromium

The fact that trace chromium was found in the area of the second '7' can only be another positive indicator that work was carried out on the plate. EDXRF and Raman spectroscopy have shown that trace elements which included chromium were found in the right-hand '7' area. 

It was present as lead chromate.

According to Professor Hall, the chromium was 'microscopic, 'non homogenous' and 'not part of an offending ink formulation'.

He further stated in his preliminary report:

“The other bands (“bumps”) in the Raman spectra are from HgS. There was
PbCrO4 in only particles in the ink and the concentration was very low. This does not jive with the “painted in” theory. From the XRF and Raman, if the color was painted in, then there would be a distribution and higher concentration of Ba, Cr, and Pb in the diamond area. This was not the case.”

According to Professor Hall:

“An infrared image of the stamps would show a response to the particles of PbCrO4 that I found surrounding the second “7 diamond region”.”

So where did the trace chromium atoms come from?

There are at least two possible ways that the trace chromium was introduced on the plate. This was detected as the compound

lead chromate.

First possible way:

1- Chromic acid is a known etching solution, plate cleaner and hardening agent. It is not beyond the bounds of reason that this was used to clean and harden the area to be engraved. Furthermore chromic acid would react with any residual lead oxide from the printing ink already on the plate in order to produce the lead chromate. Lead chromate as such may well have been used as a hardening agent.

Second possible, and perhaps more likely way that the trace chromium was introduced:

2- Residual lead chromate may have been already present on the graver or burin used to engrave the '7's, and residues may well have been left in the newly engraved area. The graver or burin used in this case to engrave the '7's may well have been used for work on earlier plates that were printed with a different ink that contained lead chromate.

We know, through a research paper on the printing inks of that period commissioned by my friend the late Don Madden, that lead chromate was used in the printing of the issues previous to the 1858-79 issue.

Fourier transform infrared spectroscopy applied to ink characterization of one-penny postage stamps printed 1841–1880 
Núria Ferrer, and Anna Vila, Serveis Cientificotècnics, University of Barcelona, Lluis Sole Sabaris, 1, 08028 Barcelona, Spain

2- Trace Phosphorus

Trace Phosphorus contaminant was also found on the right hand '7' and its origin could be phosphoric acid, a mild acid that may be used as a cleaning or etching solution and in rust removal. The Phosphorus may have also possibly originated from Barium phosphate or it may have been introduced via a possible plug that may have been inserted into the plate in order to engrave the number '7' on.

3- Trace Barium

Barium contaminant was in found in such trace amounts that the actual compound could not be identified through Raman spectroscopy. It may be Barium sulphate (Barite), Barium chromate or Barium phosphate or perhaps the base barium hydroxide used to neutralise ny excess etching acid, we may never know. It may have even been introduced via a possible plug that may have been inserted into the plate in order to engrave the number '7' on.

If further evidence was ever needed that the known and accepted plate 77 had re-engraved '7's, then the images below will provide just that. Only hand engraving will give figures of this nature.

Note how very dissimilar the '7's are from each other with some being incomplete indicating a possible lack of metal to complete that part of the figure.

http://www.1dplate77.com/1dplate77/IsthisthephilatelicJusticeaworldclassphilatelicgemdeserves.htm

Tapling stamp BA-

left hand panel

Fletcher stamp PH-

left hand panel