FPS = Firing Pin Safety. On 1911s so equipped (called a Series 80 style, after Colt's series 80 1911s) they can make disassembly/reassembly more difficult, and some say makes the trigger less crisp. They were intended to make the pistol drop safe, but other brands use a lighter firing pin to avoid the issue.

The ILS is a lock system on Springfield 1911s...or at least they had them when I owned them, they may not now. It is or was on the Mainspring Housing (MSH) which is on the rear of the frame, right below the grip safety. They are easy to replace, but you have to have a new MSH and some internal parts.

Most parts are interchangeable on a 1911...except Taurus 1911s had some parts that didn't, if I remember correctly. That isn't to say that some parts don't require fitting to the individual pistol, though. For example, if you want to put a beaver tail grip safety on a 1911 that doesn't already have one, or even just change out an existing one, you can pretty much buy any grip safety but the part may or may not "drop in." It might require some fitting for an exact fit, and/or correct functioning.