For those that haven't been able to sleep, I hit up the STMFC list and got some more input.
It looks like I'm dealing with some sort of passenger car type control valve (probably not electric) which I assume replaces the regular AB valve. Pictures from the other side of the car show a larger main reservoir in addition to the normal one which would require some sort of additional control. There's also a few other types of control valves used on passenger cars that freight cars don't normally have.
Also, in regards to the sill tabs, sort of a duh moment. The cross bracing on the under frame was most likely moved in order to accommodate the longer wheelbase truck.
When I get access to my books again, I'll do a search. Until then, if any of you passenger car experts have a guess to what type of valve this might be, that would be much appreciated.
Jason