This is most likely an Exakta external mount (sometimes called the outer Exakta mount).

While the standard Exakta mount is an internal bayonet/flange style mount, some larger lenses used the less common external version. The features you describe—especially the three inward-gripping tabs and the small locating pin—match examples discussed by collectors for large Meyer/Pentacon telephoto lenses.

So although this lens model was often sold in M42 or Pentacon 6, your copy appears to use the Exakta external mount instead.