It is indeed a hanging tank. The differences you've seen are that both HP Snyder and CWC produced the fastback frames that used this style tank. They're very similar but do not fit between the different manufacturers. I believe the CWC version had the vertical slits and the Snyder tank had the horizontal ones you see here. Being Snyder would suggest that it was a Rollfast tank.
Good piece; a lot of people could use this, I'm sure.