Many years back I was n a gunshop in the foothills of the mountains in Franklin County TN. While there a gentleman came in who had been /deer hunting up in the mountains. He had found a fired 3" 16 gauge shell on the ground & picked it up & brought it in to show. It was a Remington SP shell & marked on the side 3-1˝-7˝. Apparently Remington had put some out for testing but never marketed them.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra