As stated many, many times before S&W is in the business of manufacturing and selling guns, not making collectors happy! In this endeavor, and trying to make as much profit as possible, they always used ALL there stock of boxes, papers, cases, and other things for the guns. I am sure not all the different barrel lenghts of the different presentation cases were used up at the same time. So the switch to a new case liner type was over a long time period as the old "stock" was used up! Doc44 wrote the styro bottom/foam rubber top type liners were used until 1974. I have found through shipping dates this type was used through 1975 and the thin plastic type was started in late 1975/early 1976. I am also sure there are collectors out there that have documented guns that will dispute these dates.
IMHO I agree with Gun 4 Fun on the shipping time frame for your gun which would make it a little too early for the plastic liner type case. If your case liners are in good shape, use it! I restore these case liners and have never found a set of the styros in good shape and a lot of the plastic type are cracked and have holes in them!
jcelect SWCA#LM723
