I don’t know any specific parts numbers, but someone is currently selling a Planet Ocean bracelets with an 18mm clasp that has been retrofitted:
https://omegaforums.net/threads/ome...acelet-with-bond-007-adjustable-clasp.122521/
One of those is the clasp that came with the Spectre watch (same as the 300 MC clasp, just different branding). That is definitely an 18mm clasp. So it looks like it is possible. I think you will lose the divers extension capability, though. So I think such a beast does exist. And I believe others have added 18mm adjustable clasps to their Speedmaster bracelets. You might want to search for some of those threads as well.
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