Hull Number Decoding

[Rybones]

I am re-coring my transom and in the process have to replace the old "Florida Department of Natural Resources, Division of Law enforcement" (Now FWC) sticker with HIN that is on it because the original Stamped HIN was partially obscured. Yes, I'm having a permanently affixed plate made for it. However close, my HIN doesnt exactly follow the conventions mentioned in the message string found here and it has me curious what it means.

it is "SECF0373M74C".

I dont understand what the "M" and The "C" in there.

Hull#373?
March
1974

Is anyone able to figure this one out?