M-21 pumps?

M-12s already have frame reinforcements front and rear, albeit no points. Extend the rear reinforcements....reverse the order of model digits....12 becomes 21...

Oh, and M-42s have them, too. The sideplate would be a minor complication nowadays. Lessee....42 divided by 2 is....

Sam (coming to you from the Bermuda triangle, Area 51 to be specific)