Called my brother and he remembered.
It was the silver paint transmission shops used to spray on rebuild transmissions to make them look "new". I dont know if they still make that stuff. He also said you could spray it onto a glob of grease and it would dry into a shell over the grease.
My brother said quite a few of the hill climber guys at the time (early 1970s) didnt use a head gasket, just that silver paint stuff. The heads supposedly didnt leak and they got a little higher compression.
I wouldnt try that, but at that time 1950s bike parts were probably available at the local junk yard...
Lee