[Grant Bowker]

WRT Brian - no guarantee but it can't hurt to ask..

The levers 596973 and 596974 that the cables attach to inside the drum are standard Chevrolet parts (the numbers weren't deleted from the GM catalog until about 1979) so if we can find what else the levers were used for that might be a source for "close enough" cables. A GM Parts Wiki search says the levers (left and right) were 1937-1952 1 Ton models JE, KE, AM and BM. Also 1937-42 Models GE HE KF AN and BN plus DS and ES.
These models translate as (source: Chevs of the 40s catalog):
GE 1937 122-1/4" WB 1 Ton Truck
HE 1938 122-1/4" WB 1 Ton Truck
JE 1939 122-3/4" WB 1 Ton Truck
KE 1940 122-3/4" WB 3/4 Ton Truck
AM 1941 125-1/4" WB 3/4 Ton Truck
AN 1941 134-1/2" WB 1 Ton Truck
BM 1941 125-1/4" WB 3/4 Ton Truck
BN 1941 134-1/2" WB 1 Ton Truck
DS 1946 134-1/2" WB 1 Ton Truck
ES 1947 137" WB 1 Ton Truck
Many of the listings mention 14" brake drums.
I suspect there were other uses in addition to the above given the 1937-1952 mentioned in one listing, guessing in the 3/4 to 1 ton range.
Of course, this only gets us the attachment to the lever inside the drum. It doesn't ensure the anchorage to the drum, cable length, attachment at the forward end to frame and actuating mechanism will match...
Levers 596973 and 596974 were C8, C8a, and C15. Not C15A or any of the heavier CMP as far as I can tell. From the outside the cable may look the same but there has to be something different about the attachment to the lever. Diagram from the parts list attached.