I have a chance to buy a old Colt AR15 SP1 with a pretty low serial # it is under 11500 in good clean shape. This is not normally something I get to excited about but is this worth anything significant from the low serial #? I have an pretty old one that was my dads that has a 130,000 range serial but this one is much lower and I think from 1966? Any suggestions or thoughts would be appreciated. Thanks PG