Colt 1991 question

Looking at a used 1991 via pictures and noticed that the frame rail above the slide stop notch is cut away ala a Browning High Power. Is this an owner modification or the way the 1991 is made? I saw many 1911A1s in late Army service with the rail cracked at this point and often thought that a simple fix would be to just not have the rail at this area.

To answer my own question it turns out that this is factory original done after problems arose from the 10mm frames cracking in this area.