Omega Speedmaster 3520.50 Bracelet Fit & Options

Sorry for the very late reply, but that model (Speedmastet Mark 40) uses an 18mm bracelet but with quite chunky and tall end-links due to it being a 775x based movement.

It needs the 850 end-link and I think the correct one should be the 1562/850 here: https://www.ebay.com/itm/157633166343

If you don't mind not having a perfect fit, some bar-end type 18mm bracelets will work too, but the height of that watch case is such that most 18mm bracelets won't be a direct fit unless they have that 850 end-link.