There were two versions of the Zep. The earlier bikes (36, 37) had the parallel bars with the narrower tanks. I think the bike CCR posted is a comet but I am not the expert.
I think this tank would fit a 50s era Hawthorne/Rollfast frame, but the gills on the front look different than then ones I have. You might try posting it on the CABE...maybe someone over there can help.