Identifying 6/71 Cases

Well, then other than the difference in ribs across the inlet (as you've pointed out), and assuming that the extra ribs have not been machined out, measuring the case thickness at the inlet or outlet would be the next best answer. Judging by the small photo I have of both cases with the end plates removed the small rotor cases are upwards of 1/8" to 1/4" thicker.

I've also attached an article you might find useful, if you don't have it already.