Here's some info that may help you out:
3) All in this series had the 5 panel Superior Door.
5) Build date for series 20500 - 21499 is 1952.
6) Yup - that's original paint. Consider also Microscale 87-185.
Mainline Modeler - 9/85, 11/2001, 12/2001
RMJ - 10/1998
IIRC, the Mainline articles are your better bet - the RMJ article was mostly
about a Duane Buck customization of an Intermountain 10' 12 panel car, but
might have had photos of the 10' 6" car.
Best starting point - C&BT 10'6" 12-panel car* with 4/3/1 ends. Throw away
the details and replace with appropriate details from Plano, Branchline,
* At some point in the past there was discussion of Branchline bringing out
a 10' 6" 12-panel car. I'm not aware of this happening, but if it did, it
might well be a better choice.
Ping me if you need more.