Stock bikes in 1991 would have had this kit:
Dura Ace 27.4, American Classic 27.4 post, or a Campag one.
Maybe use a shim and be done with it? Group wise you can always put 11s chainrings on a 'period correct' chain set. I've dreamt of doing a build like this myself with lowlowlow profile rims.
@Dammit started
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Stock bikes in 1991 would have had this kit:
Dura Ace 27.4, American Classic 27.4 post, or a Campag one.