I found this in a box of misc stuff at a shop this weekend.
It's 3.46" Long by .995" wide. The opening at the top is .535". The "nub" at the bottom is .808" across and .358" from the bottom to the shoulder.
That little tab at the top swings out. At first I thought it was some sort of extended shell holder, but it doesn't fit a standard RCBS or Lee shell holder.
Any thoughts on what it is?