Sometimes in manufacturing parts, to cover all applications, the choices can be a little daunting. The harmless little Firing Pin Stop (FPS) is just such a part. Below, I'll attempt to break down the part numbers in a way that will make it easy to select the right FPS for your 1911.
There are a number of different Firing Pin Stops (hence FPS) to choose from. You can instantly reduce that number by half by deciding on whether you want a carbon steel FPS or a stainless steel FPS. The stainless FPS have a "-S" after the part number and the carbon steel does not.
That narrows it down to 4 basic part numbers - HD-205, HD-206, HD-207, HD-208
The HD-205 and HD-206 are for 1911s having no firing pin safety in the slide, also called "Series '70 slides" by many.
The HD-207 and HD-208 are for 1911s having a Colt-style Series '80 firing pin safety. If you aren't sure, you can readily tell by looking at the underside of your slide for a round safety plunger that is visible from the bottom of the slide, located just in front of the FPS. The plunger is approximately .180" in diameter.
Once you have narrowed your choices to Series '80 or not, you have to choose between caliber and slide length.
The HD-205 and the HD-207 are for .45 ACP Government Model length (5" barrel) pistols
The HD-206 and the HD-208 are for any caliber and in and shorter model than Gov't. So it fits Commanders, Officers ACP, Defender, etc. in .45, 10mm, .40, 9mm, .38 Super calibers. The difference is the location of the slot in the slide where the ejector lives. If you refer to the picture, you can see that the "all other" ejector track on the bottom slide is closer to the centerline of the slide than the .45 Gov't Model on top.
So a couple of examples to help you - On a blued Government Model sized .45 ACP caliber pistol without a firing pin safety, you'd order a HD-205. If you had a S/S Gov't Model in .45 ACP with a Series '80 safety, you'd order an HD-207-S.
If you had a blued Series-80 Commander in 9mm, you'd order a HD-208. If you had a blued Commander in .45 ACP, you'd order a HD-206.
Next you may notice some firing pin stops with "LPA" in their part numbers. The are shortened at the top to allow fitting to pistols with the "LPA" adjustable rear sight cut that Springfield uses on their guns. It will also work for Bomar rear sight applications.
Series II Kimbers with their "Schwartz style" firing pin safeties can use the Series '80 FPS.