You might have a look at some of the old catalogs to get an idea. Here's one from '62: http://www.old-omegas.com/sea62en.html
eBay might turn up some watches for sale with original straps---that'll give you some idea too. I suspect you're right in your guess. If you don't care about actually obtaining an original strap with an original buckle (which could be difficult to do without actually buying a second watch that comes with those items), but are rather looking to recreate the look, then it should be fairly easy and not too expensive. For those older vintage Seamasters, I'd make sure to pick a strap that's 3mm thin or less--I've got mine on a 2mm strap and it's just right. ("Ultra thin" is a good phrase to search for.) Just my opinion---but those pieces are so elegantly slender, you don't want a strap that detracts from that.
