Is there a reason they're nylon? Could the brass wear too much into the frame? On the front at the header bow mine were crumbling. So i switched to brass. Dont remember where i got these and they're not an exact match, but here are some pics.
Old/new

I.D.

Test fit.

I see no reason why those brass ones shouldn't work fine. Maybe the factory used nylon to prevent a possible squeak?

Yeah, or cost maybe, who knows. I put a dab of graphite grease just in case. Will see.

Are new nylon bushings available? From whom? Thanks.

I couldn't locate any. Internet or local.

I'd bet it was rattle or squeak issues.
maybe machine some from delrin, may work better than nylon

The nylon bushings are not flat like the brass pieces. So it's a no-go finding any direct replacement. If i had a mini lathe i would've tried making some.