Original owner watches you typically see as is or rough, filling down a 506 knowing it'll fit and adding a 1035 off a Seamaster or something to increase the value is something you'll commonly see a dealer do.
If the owner really did buy it in 1966, that was ~50 years ago, so he'd have to be in his 70s at least, I don't see him fishing eBay for end links to modify and a spare bracelet like that.
Its possible a watchmaker did it at some point over the years but it just raises the question for me as its something typically done to maximise potential sale value on a Speedmaster, once again not something a 70-80+ year old original owner would be expected to do.
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