Globe Electric lists an 537 in their catalog. It's probabaly not a Signal-Stat, but then those were sold under many brands names back when they were in common use.
They also list a heavy duty electronic flasher, 3 terminal alternating for $15.08 US. Not sure it's absolutely period correct, but it will work if function is essential. We used that flasher in the 80's when the 537's crapped out.