I am pretty sure that the .45 sw99 only came with two backstraps, small and a medium. After spending some time trying to find the "missing" 3rd .45 backstrap, I determined that they only shipped with 2 and so I had them both. FWIW, The difference between the two that came with my gun is almost unnoticeable, only really adjusts trigger reach, not width or swell.
Good luck