91 is rare? Why so? ...Why are they rare?
A fairly small number were produced, for P-series guns - if my memory is correct, they only made the P91 for about three years, before replacing it with the P944. The serial number range suggests about 25,000 per year, so maybe 75,000 total produced. I think the only P-series gun that was produced in smaller numbers was the P93, with about 25,000 total (again IIRC).
The P91 was the last of the "full size" P-series designs, and is quite similar to the earlier P89 & P90. I found that a P89 top half would fit my P91 frame (with a tiny bit of file work on the bottom of the slide, to clear the ejector), so I can also shoot 9mm, using 9mm mags. And I discovered that a P90 top half will fit if you want to shoot .45 reloads: quite a bit more fitting involved for the barrel to clear the frame, and cartridge length has to be short enough to fit a 40 cal magazine (but there is published data available at that length). In some ways, the P91 is the ultimate "Ruger convertible"!
As always FWIW, IMHO, YMMV, "don't try this at home kids", etc., etc.