Looking for 1964 Omega Speedmaster watch band

It should be a Ref 1506 bracelet with #16 end links

This chart outlines the options:


It's mostly the same as the Ref 1039 bracelet with #516 end links, which was used on later 145.012s and is a bit more common, I'd probably be happy with either bot the 1506 is the exact reference it would have had originally.

They do come up on eBay periodically, but they're not cheap, they're also a bit of a hair puller, and a bit flimsy and fragile compared to later bracelets

This is almost correct, the #516 end-links are almost the same as the #16s and will fit, not cheap though as you can see:

https://www.ebay.com/itm/178175308803

Honestly for most people it would be minimal, people tend to look at a 321 Speedmaster Pro as a sum of its parts when assessing that, the 1039 is close enough that for all intents and purposes most wouldn't care if they're intending to wear it. If you're intending to sell it, you're better off leaving it as is, or selling it on strap and selling the bracelet separately as you won't recoup the cost of the flat-link bracelet in the sale.

You could follow some sales on both 1506 and 1039 here, they do come up:
https://omegaforums.net/forums/watch-parts-accessories-for-sale/