This mystery is fun.
As I indicated in my complicated way of explaining, I do not think it is related to the Peninsular War. My guesses go toward another (romantic) explaination.
We have here a typical problem which requires assumptions for solving. I may remind in this place to Occam's razor:
http://en.wikipedia.org/wiki/Occam%27s_razor
So we should keep the list of assumtions small. What is the simplest, most unspectacular solution?
I like the statement from JLDoggett about the two hands. It makes sense. Also I start to think the engraving was made in UK. The style is Blackletter
http://en.wikipedia.org/wiki/Blackletter
This is likely to be made in UK, maybe later than 1808. We can assume this needed a specialist to engrave. So in the workshop the letters were done by the specialist, while the other decoration was done by a less experienced person.
I think the key to the solution is the word L'AMYTYE. Sounds definitely from latin language origin. It was pointed out Old French. I think it is a code language of two English speaking persons, using latin styled words for coded communication. I understand this was rather common at this time, to communicate with codes. In an environment so precisly described in the novels of Jane Austin, this box can be present to a beloved man. It reads "Remembered Friend" as Silverly pointed out. Just with difficult to read letters and latin language (French, Italian, Spanish, Portugese....) based words.
I think this would be a simple, undramatic solution with little assumptions.
Regards
Jörg