I just saw my first Frankford Arsenal rollmark a few days ago at the range. A guy about my age (mid-30's) had a well-loved, 16" A1, no-forward-assist upper. It was his boyhood rifle that he had recently obtained from his dad.
He evidently had a lot of nostalgia towards it, FWIW. I left before he fired it so can't comment on that.
Before that, I had never heard of them.