The 16800, 168000 and 16610 all using the same bracelet. Either 93150 with 501B end ling or with SEL end link and it is numbered 93250.
The same 93150 bracelet can also fit the no date Plexi 5513 sub and all we change is the end links to be 580.
Now the EXPLORER II is also using the endlink 501B, so bracelet 78360 (from EXP ) will also fit the SUB

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Like a LEGO bricks
1.) buy a 16610 (or 16800) from the late 1980s/1990s and and have an SEL bracelet come with it. - YES
1.a.) Have the option to put GMT/explorer2 bracelets you mentioned on it. (Though not sure the difference between each?) -- YES
2.) buy a 1680 and it’ll come with a “folded” bracelet - YES
(if the watch was kept original)
3.) buy 16610 even from 2009 - YES
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See the photo. tried to compare both clasps. as you can see the SUB has a longer clasp .

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