At an estate sale, and it was a very out of place find. I was the first one in the room and aside from a couple of nice older 12" C-clamps there wasn't much similar or even toward that time period. I picked the room over and grabbed a couple of other smalls, but exited in a hurry to go to another sale just up the block.
I stopped back by later that evening to give things a more thorough look. There was a beat up two drawer file cabinet sitting to the side covered in trash, and there they were in the bottom drawer. Nothing else in the cabinet. It appeared as if they were going to junk the entire pile, however they were happy to sell them to me once I asked / showed interest

"Oh, those nasty things covered in oil?"
I'm still happy with the transaction, though...