From my Dad & Uncles (3) experiences during WW2- USMM’s were generally given blanket draft exemption as long as they were actively sailing and didn’t spend more than 30 days on the beach between ships. The FBI frequently came to a seaman’s home of record on the 28th day and were “reminded” to ship out…This happened to all four of my family members- they generally shipped out on the 28-29 day, except when they were UFFD from injuries sustained after they were torpedoed- in which case my Grandmother starting throwing things at them…
During the Vietnam war, at least two guys I know of (both two Deck Officers) that were drafted into the Army (One of them was one of the best guys I ever sailed with- the other one of the lousiest). The draft deferment for was applied for by getting a letter from the company and union stating that you were actively shipping out. The former didn’t know this. The latter was such a louse that the company “lost” his letter and he got drafted and was actually assigned to Army stevedoring in Saigon- where his former shipmates taunted the shit out of him.
Another family member was classified 1A- but was in Calhoun-MEBA, all he had to do was prove receipt of getting a license- and APPLY for a NR commission (which of course was more often than not turned down), also as I recall; he stated that USL easily gave the required letters to those that shipped with them for more than 4-6 months yearly.
So there you have it- different methods- but the result still the same-