Nope not self-modified but direct from (friend Jerry Grant RIP, 1st Indy driver to go 200+ MPH) Champion Spark Plugs (Toledo) from a handwritten Indy logbook. When I asked Jerry if I could get a couple of these N-S 501 plugs he asked me where the H377 I found the number as he had to go back to handwritten Indy logbooks to find them. I told him an elder factory engine builder for a company I worked for told me to try and find them for a race engine I was building, Jerry found a few boxes left and sent all of them to me. This was back in the early/mid '80's.