The early SMG 60's had the serial # located on the bottom of the trigger guard and later models had it on the left side above the trigger.
Back when the 68 Specials came out Tippmann would convert your SMG's into a 68 Special for $100. There were a lot of SMG's that were converted. When Tippmann did the conversion they did not remove the SMG tag with the Serial Number so even though the tag says SMG it is a 68 Special.
The easiest way to tell a SMG 60 from a SMG 68 is that the feed tube is held on by a cap screw on the left side of the 60's. The 68's the feed tube is held on by the barrel. The barrel has to be lessened up to get the feed tube out of the 68's
For the stripper clips to feed they must have paint in them or if you want to see how the clips work with out paint, you must find some way to block the barrel. The way the indexing of the stripper clips work is it uses back pressure to index them.