glock 36 trigger pin

As I recall, glock pins go in from right to left, and out from left to right, so the way the pin is pictured with the close end groove on the left would be the way it sits in the gun (looking at it from the rear).

https://www.gunpartscorp.com/Manufacturers/Glock-33268/36-36384.htm

Here's my guess.
1. The groove allows for immediate identification of this trigger pin as being for a Glock 36, negating any need to compare it to the other Glock trigger pins.
2. I'd try installing the pin with the extra groove on either side, but suspect it'll have the easiest, most success with the extra groove on the right to not have any chance of getting hung up on the slide stop during assembly or disassembly.

I don't know much about Glocks innards but similar parts I've dealt with that groove would be on there so as to be retained in place by a spring of some sort.

The end of a spring sits in that groove.