Must be an old conversion. I've never heard of it. I do Know that Knight made a rifle that used plastic 38 cases for use with large rifle primers. It must be a conversion for a Knight plunger gun. Did the holders look like the picture below?
EDIT: I just did a search and found a post on a large rifle conversion. It's a little over a year old.
http://www.huntingnet.com/forum/tm.aspx?m=1637435
ORIGINAL: Rebel Hog
Well, when I bought theWolverine I gotit from Cabelas anditcame with cap set-up and then Knight came out with the large rifle primer conversion and I bought itthrough Cabelas. Then they came out with the shotgun primerconversion and I left it with the Large Primer Conversion. Ihave 1500 large rifle primers and it's all I need really!