Identify rear truck mount that is missing/broke.

Specific model numbers and/or photos would help.

Hi C W, It's a Weaver PRR L-1S 2-8-2 steam loco #1138.  Can't find a model number... made in the "90s?" 

It's definitely missing since I see another one on line with  a "stub" sticking through the curve in the truck plate and with a C-clip holding truck on.

In this next picture you can see the post and the curved slot in the rear truck plat.  

So what is this piece called... do I need it to operate fully since the rear trucks don't bear any weight and are pulled around the track?  Wouldn't his be a bigger problem if going in reverse?

Thanks for your help.

A spring normally goes between the wheelset frame and the locomotive frame along that broken stud. To me it looks like you could probably drill that out and retrofit a round head bolt with a spring slipped over it to fix it. That would keep pressure against the wheelset to the track.