As far as I can tell from the parts books, the 1-1/4" master cylinder used on the C15TA is the same as the master cylinder used on C8 to C60L (C60X is different) part number 5450260. The parts book gives the Chevrolet part number for the kit as 603646.
Al below is less certain or guesswork.
I suspect the master cylinder assembly crosses to Wagner FD-4388 and the kit to Wagner FC-3060 based on a Wagner manual published 1947 that shows cylinder diameter, general layout and mounting style. The FD 4388 is listed as for General Motors vehicles.
There are currently ebay listings for NOS Chevrolet 603646 though I'm not sure I would want to replace old rubber with nearly as old rubber.
I suspect that a generic Chevrolet 1-1/4 kit listed for about the 1940s period might work. For example
http://www.oldchevytrucks.com/cart/p...9&i=19555||||| or
https://www.chevsofthe40s.com/detail...n_3652_34.html I haven't tried this so offer no guarantee as to suitability.
Googling 5454428 does come back to GM Master Cylinder Kit. That number appears to cross to Dorman TM3630. Another possibility might be Raybestos MK33 (maybe MK3 as well?)
http://www.rockauto.com/en/catalog/c...epair+kit,1840
Based on what I found today (March 16, 2018) when looking for info on another thread,
http://www.mapleleafup.net/forums/sh...ight=raybestos I think the Raybestos MK3 is for 1" master cylinder and the MK33 for a 1-1/4" MC.