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Or the had a Harley engineer on staff. They have splined shifter issues forever!!
It's nothing new, my 80 SP lost hers in 81. Multiple others I have fixed over the years, even around 2000 when the idiots started using an allen in it. I replace the allen w/a hex head and put on blue Loctite.
It's not like it's anything new - Loopframe drum brake splined shafts were made that way. I've never had any problem with anything falling off. Don't really see the need for a slot.
If you put on a hex head, you can get a 10mm long wrench in there by going with hand up next to wheel above swingarm gap forward.