Must be a new OEM part.
My '18 Tri-Glide has the "shorty".

My thought is that any longer bolt/screw could work by using to two "jam nuts" and exposing a portion of the end threads to mount to secure the seat to the fender.
Next time I'm at the hardware store I'll get the parts and give it a try.

 

2AMguy that will work also. I used a 1/4 in bolt on my 2016 Tri Glide worked much better then the short screw.

 

+1 on the piece of milk jug holding it, never gets dropped any more. I found the longer screw in my old fasteners drawer, and had a much larger diameter head on it, and I ran a couple of nuts onto it first and jammed them, makes it harder to miss with the screwdriver and easier to see. I filed off the threads where the plastic piece goes so it would spin free and won't unthread.

Here's five other threads on Seat Screw in the trike forum alone...
https://www.hdforums.com/forum/searc...rchid=46658517

 

Long 1/4" bolt works for me.

 

I did the milk jug thing in 2013 just after I bought old Shiney still using the same piece. and it still looks as good now or better

My favorite rides

 

1/4 x 20 T-Bolt for small bolt chevy. Install and remove by hand. They are long with a T at the top so it can be installed by hand.

 

Just seen there is another thread on this already on here. Sorry !

 

I quit using the seat bolt when I had my 2016 Triglide and I don"t use use it on my 2020 either. The tab for the screw sits far enough down in the recess on the body that the seat never moves. I've got close to 20,000 miles riding without the screw with no problems. Makes it a lot easier to remove the seat to adjust the rear suspension on my 2019 also

 

Will someone elaborate on this milk jug thing, preferably with pics?

 

Make a small round washer out of milk jug and use it to keep the screw on the seat while installing it.