Lobster tailed helmet circa 1640

[fernando]

Originally Posted by Victrix

...I always thought those letters stamped on the nasal guard were arsenal marks, but I could be wrong...

Yes, probably some code letter to deal with production inside the arsenal or workshop, assembly lot or the like, but not an actual origin mark to be recognized on the "market".
BTW, isn't that a letter (M?) in the nasal of the impecable example you are posting ?