In Buttweiler Auction Volume XI Number 1, he had a 4.5 F.N. Experimental based on a .22 Winchester Magnum impressed “C” headstamp, that had been necked down to 4.5mm (.177). He said it was reported to be a test design used in the development of the centerfire P90 Weapons System. Does any have any information on this test series? What other variations of this rimfire experimental exist (headstamp, neck angles, bullets etc.)?