I bought my Sherpa frame and fork used in Spring of 2010. How do I tell the difference between plain Sherpa, MkII, etc.? Or is there some code in the serial number that would tell me which Sherpa I own? It is size 610S. The seller provided the original receipt from his purchase from SJS (I wanted to make sure I was not buying stolen parts, thus asked for proof of ownership), but I have misplaced that receipt so I do not know when it was originally purchased.