Mike,
I almost feel guilty doing this but it seems that the Q had the C&S a little while before the 1920s. Pehaps I'm mis-understanding your post but the date of X December 1908 comes to mind as the point the C&S was sold to the CB&Q. (I don't recall the exact date). Or am I missing something?
I have copies of letters concerning C&S equipment lettering from the '20s when the RR was gearing up to replace the old trade mark (we call the Block lettering initiated in 1907 iirc). My understanding is that this new Circular Trademark had been in use on stationary and official documents, timetables etc. in one form or another since around the turn of the century. 1925 seems to be the earliest it was applied to freight cars and in it's earliest form seems to have been just the white parts without the black background.
The Q, in fact, did instruct the C&S to letter it's ng stock with the Box Trademark (not sure of it's official name) of the CB&Q but Rice (iirc) responded that, because they were attempting to abandon the ng it might not be a good idea to remind the public who really owned the C&S - the fear being that the argument, the CB&Q was a larger system that could absorb the losses incurred by the ng, would actually work.
Obviously Chicago agreed and with perhaps the exception of Nine none of the ng engines or cars were lettered for the CB&Q. That I am aware of. But up to that point there was quite a bit of message traffic about the matter and it almost happened.
Hope you find this useful or at least non-offensive.
Derrell