I have held off a long time after seeing the pictures - to think and re-think my comments. But the edge shots have now given me great concern. Since submission to a
TPG is imminent and they do not typically look at or consider the edge - I wanted to reply before it possibly gets covered forever.
In 1840 there were branch mints at Mexico that used hubs made "overseas". That fact is well known. These dies were French and English (Boulton- Soho) made. Both of those mints were at the TOP of the game - so to speak. Both were exporting state of the art technology that FAR surpassed the US and both were light years ahead of Mexico. (I must note that here in referring to England I am not referring to the country but the mint operated by Matthew Boulton's son at Soho). England was using Boulton mint equipment but was not exporting the technology or selling die making skills.
The reeded edge applied to production runs was produced in England and in France by a retracting collar die or dies. This technology was restricted to POWERED presses and COULD NOT be used on hand operated screw presses.
The edge on this pattern piece was not applied using either of those moving collar techniques. This implies it was struck on a collarless press and edged in a separate step using a low tech press. WHY?

In 1840 it is also well known that it was still illegal for the branch mints to use dies made outside of Mexico. The mints operated by lessors with ties to European financial interests (especially England) did circumvent the laws by smuggling dies into their compatriots. Making the dies elsewhere greatly increased production rates and profits. This is exactly why Bras de Fer did it in the first place in 1831.
The characteristic most normally associated with a Pattern coin is that they represent the VERY BEST POSSIBLE techniques. This applies to die cutting and strike both. No one advertises with poor looking or poorly made coins.
Here from the outset I have seen a poorly finished die that leaves much to be asked in comparison to the typical pattern strike of the 1840 era. The plating used amplifies the poor appearance overall since it obscures detail.
So was the plating original to the pattern or was it added later in an attempt to pass this coin? If I knew how all the other patterns of this same type were surfaced I might be able to dismiss this question.
The plating method which leaves gaps in the "shadows" of letters most often is an electro-galvanic process. That shadow is an effect of the way the electrode (the coin) pulls ions of silver out of the electrolyte (the silver enriched liquid medium in which the coin is suspended). In the area where the letters rise above the coin's surface some ions migrate to the higher elevations on the letters instead of directly to the surface (field) of the coin near the bases of the letters. This results in a much thinner plate as you approach the letters. This area also accumulates most dirt which can remove any traces of plate in actual circulation. The faster the deposition process the more pronounced the effect. This effect is seen most often on cheaply made Counterfeit 8Rs produced in the 1870s and 1880s.
The problem I see with this pattern is the date of the electro-galvanic introduction. It was known but not widely used in 1840 and the way this piece is done is very POOR. It looks bad and would make a BAD impression. Not good for sales. So I see it as an indication of a late date for the plate. Unfortunately an original pattern made in 1840 could have been plated in 1880. I know of no way to distinguish the two - unless the other pattern copies of this design ALL LACK the plate. The one place where electro-deposition was well underway in 1840 was England. But that leaves the question of why did the do such a poor job when Boulton specialized in thin plate Sheffield and amalgam plating methods?
There are other silver plate processes that could also leave the shadow as a feature of flow across the coin's surface especially if done hastily. These are earlier but they tend to make the surfaces of the coin plated look poor and they tend to obscure the fine die details. Once again the use of any plating system that disguises the surface of the coin is a BAD sales technique for a die manufacturer trying to get a contract. A sharp strike in copper is far better to show workmanship.
I agree with John that the plate would be very interesting to test. Electro-deposition leaves behind 999 fine silver and only the fact that XRF reads "INTO" the coin slightly would skew the effect slightly by reading the metal beneath. Testing numerous thick spots versus a clean underneath spot could allow a clear picture of the nature of the plate. Washes and other plating process leave traces of other metals and could be eliminated.
After thinking through the plate I turned back to the die work I see on the pattern itself. The die work I see seems very poor quality for either Boulton or the French - it is blunted, not SHARP and DISTINCT as I would expect. For contrast, anyone interested should look at any of the pattern strikes Boulton actually made - there are numbers of those in Doty's book on the Soho Mint. You can also look at the great number of French patterns made in the 1840's that are shown in many books on the mints and minting.
I can think of three possibilities at this point.
1. The coin is a simple counterfeit made anywhere for circulation, in which case, all of these "patterns" would be re-classified as contemporary counterfeits. I view this as possible it has happened before.
2. A counterfeit of a real Pattern made for numismatic fraud. This is also possible given that everyone knows this particular coin is an early reeded 8R and given the fact that Numismatic counterfeits of this coin do exist.
3. The coin is a real pattern made locally in Mexico or someplace else with odd legends and a bad reeded edge.
But why the edge - WHY REEDS?
The technology is NOT Mexican in 1840 and why would they mimic a process they did not use or have the technical capability to employ in 1840 without buying machinery from Europe or elsewhere?
That reeded edge pushes me toward 1 above not 3.
The reason is simple. If reeds were applied to sell a moving collar die press - WHY put the reeds on to look HORRIBLE? Why not just mount the pattern dies in a moving collar press?
I am not at a point where I am comfortable in making a final opinion. But the appearance bothers me.