The good question is, when did they introduce the case stamp referencing 20 world records, and were cases with the previous "7 records" stamp still in production for a while in 1928, or were they not. In 1926 and 1927, the cases still had "7 records." That said, this part is not quite as obvious as it could seem.Except that by 1928, the Rolex case stamps should be referencing 20 world records, no?
I actually sent a snap of the back to this site earlier today. No response yet.