Sourcing odd casebacks is most of what I do, so let me add a few tricks to the eBay-saved-search advice (which is right, just slow on its own):
Search the caliber, not the model. Most sellers of dead ones don't know what "Marine Standard" is - that's your discount. Saved searches for "omega T17", "omega tank watch vintage", "omega rectangular 1940 not working", and the misspellings ("omega marine standerd") catch the listings the collectors never see. The several-thousand-dollar listings you've found are priced
because the seller knows the name; the donors come from sellers who don't.
Hunt watchmaker estates, not watch listings. When an old repairer's stock gets cleared, casebacks turn up in "watch parts lot" and "watchmaker case parts drawer" listings for pennies, because nobody catalogs them. Those lots, plus the NAWCC mart boards and the parts tables at regional watch fairs, are where orphan casebacks actually live.
Post in this forum's Wanted To Buy section too - there's a 2021 Marine Standard thread in there, so these do circulate among members, and the person holding a spare back is far more likely to be here than on eBay.
And measure before you buy anything: get your watchmaker to give you the exact back dimensions and the seal/groove style, because the 3635 and 3683 backs are close cousins, and a "Marine Standard caseback" listing won't distinguish them. Buying by the numbers beats buying by the name.
I break vintage Omegas for parts as a hobby - if a T17-era donor ever crosses my bench, I'll sing out. Good luck with the hunt; that's a watch worth the patience.
