What Is The J M On This 344 Block

Would this be engravers initials? How many times per sheet would this be seen?

Plate engravers, finishers.

Usually the initials were added for each
touch up....thus sometimes you
see the long string of initials.

I have heard of a list (cant remember where,
you should google search maybe???) where there
are names for the initials.

Check this out:

http://home.earthlink.net/~davinod/Initials.htm

Excellent link, typesetter! The web has some wonderful tools, and this one I bookmarked.

I have never seen a more comprehensive list edited into one place, although I know that the "Marginal Markings" group in the BIA/USSS accumulated them intermittently over time in the US Specialist.

Yet, for all its completeness I am not seeing a listing for the JM mark that stampcrow is showing. I'm probably missing something, but maybe the list is.

I never seem to hit on the correct terms when I google search.
Thanks for the help folks!