Local Rolex-trained watchmaker told me you can smoosh 'em tighter between your fingers and then test fit.
Something I did, and if you ruin your end links it's not my fault, was to mount the end link and the spring bar but not the bracelet, and use smooth-jawed pliers to squeeze together at the corners and then finish by bending the tabs up.
Now! All this being said, if you buy a brand-new set of 633 end links from Otto Frei... the fit on a Speedmaster is nearly perfect. Yeah, costs money, but you'll be a lot closer to nirvana.
Tom
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