There are three different tank plates. The earliest is the short style. This one attaches to the frame at the two mounting bolts only and the bracket itself does not extend beneath the seat hinge. The are two versions of the later long style. The earlier type has a smaller opening that fits the neck of the plastic tank. The later has the larger opening matched to the larger diameter metal tank filler neck. Both extend under the seat hinge giving, in effect, a four-point mounting.
Most people prefer the plastic tank because it doesn't rust. That comes with a couple of tradeoffs. You've seen the first. I suspect that's the reason these were superceded by the metal tanks midway through 1972 during the K1 model run. The caps used on the metal tanks have a real vent shutoff, while the plastic caps are kinda iffy. This could be an issue if you transport your bike in a closed vehicle.
Either tank will fit any CT70, ST70, ST50 or ST90. The smaller tank bracket won't fit the metal tank, but the later style can be sourced easily enough. I believe it's the same one used from `72 through to the present-day Chinese knockoffs. The only part not available new is the rubber grommet used with the plastic tank.
