The A65/A50 cross strap has a welded on cup to fit over the rubber sleeve that slides up onto the pointed rod that is welded to the underside of the frame tube (as in Adams photo). Both the big 4 gal tanks and the smaller 2 gal tanks have a strap. The 2 gal strap is flatter than the one used on the 4 gal tanks.

In both case the rubber sleeve isolates the tank from contacting the frame.

Without the strap the tank would be quite unstable when full with only the single top bolt holding it.