One of the great things about restoring 1940's and 1950's bikes is that some of the 1950's high-end stuff moved down to being shipped on low-end bikes in the 1970's, I believe (please someone correct me if I'm wrong.) I'm thinking perhaps about Stronglight steel cottered cranks, Rigidia Chromalux rims, Weinmann 500 brakes, and a few other "mid to high end" 1950's parts that managed to stay in production when the companies making more esoteric stuff went bankrupt ...
Bikes should be part'ted up to be ridden and enjoyed. One way to do it is to build a "theme" bike ("japan", "italy", "france"). One way to do it is to build a "this is what I consider to be the best of everything" bike. Then take it on a ride to show off your beliefs. One way to do it is to build it as, "this bike matchs parts quality perfectly to frame quality ~ perfectly balanced!". Another is, "this bike implements a particularly fetching color or pantograph scheme." Most of these ideals can be implemented with parts from ANY era.
"Period Correct" is really just a minor subset of what's possible.
If you can't be "Period Correct", how about "Functionally Correct" or "Mechanically Correct" ??
When the NOS or vintage parts run out, you might select a modern copy of the original parts. For this reason, sugino bottom bracket cups, nitto stems and engraved bars, and MKS pedals continue to be produced and sold in great numbers, to support this market. Fluted seatposts are gone but aluminum tubes are still available, and 1-bolt and 2-bolt microadjust posts are avilable from asia that are not unlike nuovo/super record and chorus. Velo Orange just brough back the polished 700c rim. I prefer sometimes to buy modern copies and strip the logos and anodizing to make them more generic or more similar to "period correct".
If you value a supply of replacement parts, I encourage you to patronize the manufacturers who are keeping fresh supplies of vintage clone-parts in production.
- Don Gillies
San Diego, CA