Rolltop desk with mystery component

Hi there,

I was just given an antique rolltop desk by a friend whose father recently passed away. He tells me that his dad bought it in the 70s and by then it was already at least 50 years old. It has a mix of screws (fastening the upper and lower sections), pegs (the back panel between the drawer pedestals), and dovetailed drawer joints. Everything about it seems pretty straight forward except for one component. On the inside of each "S Curve" panel is a strip of wood with rounded ends. On both ends of these pieces are what appear to be wooden wheels/coasters sandwiched between them and the panel. It seems like there was something that slid along these supports. I've attached a couple of pics for your inspection. Any help identifying their function would be greatly appreciated!

 

Looks like a nice place to place the envelopes you received in the mail that day.

 

I called them "letter bars". (Incoming and Outgoing?)