There is no "factory" reason for a different lifter. PO(previous owner) may have had many reasons to put in a different lifter. Not many would be good ones. If you are pulling it down to rebuild then you are planning new lifters and cam I would expect. If the cam is alright and the plan is to reuse it and you have kept all the lifters in order then you should be OK. That is a lot of if's. The single lifter might have been used or scored or...whatever it should have been replaced as a set and might be a good reason to look at the cam real close. And at the lifter bore in the block. It is worn a bit different from the rest. May have been put in as a temp fix and now you get to do the proper fix, use a new set. Maybe the original pushrod failed, PO put 1 new lifter and 1 new pushrod in and called it a day. I would not recommend you do the same. Weight difference could be just a different manufacture or vendor or it could be something else. I have not seen a "shouldered" lifter but that is just me. Others might know if there was a Part number change....but only P/N change I know about would be for hydraulic verses solid. You can put new lifters on an old cam. You cannot put a new cam in with old lifters. Too many changes to materials in the last 60 years.

Steve H