Manuel,

that's my idea entirely!

Mine came on a period leather rallye on almost unworn, unscratched, Autavia Rindt.
The buckle is not the highest quality and has a little oxidation.

Now Heuer in the 60's would have had these buckles produced by an outside supplier.
The "sun" and "beta" are likely manufacturer's stamps. Possibly they had a batch made by "Beta",
decided the quality was not good enough, and then had another batch made by "sun".
Then maybe for another run they went back to "Beta".

Then there's MichWir calling a "banded" pins fakes. Likely just another production run.
Quite possibly ordered by Heuer, or Heuer USA?, similar as Rolex bracelets.

I think the sequence in my picture may well be correct for the buckles shown. Beta, Sun, B.
But that's just me!

Jurgen