I have no catalogs that date that far back, but I can throw in a few points:
I believe the Cobra, introduced in 1951, was first offered with Coltwood stocks. (The original Commander, with an intro date of '49, did, too, but that doesn't confirm they were also used for the DS at that time).
Severn's book mentions that there were two types of plastic stocks used: the first had a dark reddish hue while the later ones, termed "Coltwood," were dark brown.
I know that they switched back to walnut no later than the mid-'60s and it was probably much earlier than that. Mid-50s? Late-'50s? Early-60s? I don't know.