No, I know of no site that would provide the informaton you want. However, except for minor variation in length, most of the “R - P” headstamps are quite like the W&M drawing. There is no consistency over the years 1960-1970’s for the length of the dash. It varies from one caliber to another. That being said, the early 1960’s rounds do seem to me to be somewhat longer than most of the later ones. But, I still think, based on when I started to notice the very short “dash”, that it is intended to be a “dot” not a “dash”.
Going back to your original premise of being able to date a cartidge by the style of the headstamp, the answer is “Yes”, you can, but usually in only broad groups of years. Except for most military headstamps which are usually year dated, most commercial headstamps are used for a number of years. For instance, the predcessor the the “R-P” was “REM-UMC”. That headstamp was used from about 1912 to 1960 with VERY little variation. Other factors such as primer type or bullet material or bullet style will more accurately date a cartridge than the headstamp alone. Of course using the bullet implies you have a "live"round, not just an empty case.[/quote]
Thank you – this is helpful.