My thinking is these are on the tough to find area if brass. I bought one a few years ago and had not seen another until a couple months ago, the one Doug has now. I would guess they are test pieces from 1973. Do a test on junk metal before you do the actual run in silver.
Of course I was not there then so can not know as a fact. Just all guessing as we never see the brass but do see the silver pop up now and then.