There are many designs of trigger pins not even counting different diameters. All need some form of anti-walk device so the pins don't work their way out of the receiver.
1. One groove near an inside receiver wall. Use as the trigger pin and held by either side of the hammer spring.
2. One groove in the center. This is the hammer pin held in place with with a wire spring in the hammer.
3. One groove in the center, one groove near an inside wall. This is the most common "mil-spec" pin which can be used as either the hammer pin or the trigger pin and nether care which way they're oriented when used with a mil-spec trigger.
4. One groove near each inside wall, none in the center. Used as the trigger pin for better reliabilty with #1 pin for the hammer. Direction doesn't matter but trigger and hammer pins then aren't interchangable.
5. Three groove pins to do the jobs of 1 and 4. Reliable when installed but more difficult to assemble.
6. longer pin with groove on each outside wall for circlip. Easy to assemble/disassemel but easy to lose parts.
7. straight pin with inside threads on each end. Uses pan head screws on outside. Provides both anti walk and anti rotation. Easy to lose screws.
8 bare pin with exernal springs mounted to outside of receiver. I can't guess why.
Various aftermarket trigger makers use the grooves for different things or provide custom pins. Jewell for example provides a spring which will fit on the #3 style pin for the hammer on the right side. I don't have a list of all the possibilities. The logic of the mil-spec #3 pin is that it can be field stripped with just an FMJ bullet tip and the direction or which pin is which doesn't matter.
Use Google images with "ar-15 trigger pin" to see examples of all of the above.
