Charlie Vlk wrote

I haven't cross-referenced them with the NP equipment, but I know
that the CB&Q built flatcars using a NP design that, at least
generally, looks like the models being discussed here.

Yep. Aaron has photos of his model on his web site, and mentions
the NP/CB&Q/SP&S cars. The CB&Q cars had one minor difference that
he points out.

W&R did these in brass years ago, as well as the rebuilt bulkhead
cars. Byron Rose was very annoyed because the W&R cars used oversize
stringers on the underframe!

Ted, when you do those NP single sheathed 50 footers, I will be one
happy camper! One thing lacking on 1950's layouts is enough of the
1920's 50 foot cars...

Tim O.