I haven't been able to come up with a method of dating , but just from my own experience. The very first ones were mfg. in the USA and used Douglas barrels , and had pewter furniture and double wedge pins, some were sold as kits and some assembled. IIRC the next gen. were produced in Spain and had brass furniture, single wedge pin, some even had a slow twist and were deadly accurate.The next gen, AFAIK and I'm sure someone can correct me were bought out by Traditions.
Cheers nessy.