Filian-ga: I just posted this but accidentally sent it to the 
Miscellaneous category, and though I'm sure you could find it --
finding things
is, after all, what you do -- wanted to post it in the same place we
started from, alongside Markj's. Please officially answer the question
about Boatswain's Mates so I can pay you.

Many thanks,
McCook

Hello McCook-ga!

"BMCS" means "Senior Chief Boatswain's Mate" according to this page
from the Coast Guard:
 
US Coast Guard Enlisted Rank/Rating Insignia 
http://www.uscg.mil/images/graphics/ranks/enlisted.htm 

SEARCH STRATEGY:
Coast Guard and Navy ranks
Coast Guard and Navy BMCS acronym
Coast Guard and Navy BMCS
Boatswain's Mate BMCS


It's been my pleasure to answer this question for you. Thank you so
much for your kindness and generosity. The only question I now have is
why they call it "BMCS" instead of "SCBM" or "BMSC"? :)

Thanks again!

Sincerely,
Filian

Swift, sure and most important, right. Thanks, Filian, and apologies
for supposing you were a "he"!