This is the first I've seen mention of the .257 STW here!
I know of a gunsmith in W. GA (Woodbury, GA -Meriwether Co.) who has built a lot of them. Last name is Fuller.
I don't know of any source of data, but if you can get in contact with him, he could put you onto some.
His shop looks like a dump till you go inside. Looks like a "buggy barn" that hasn't seen a hammer or nail in 75yrs. Has a logging chain through holes in the door and a master lock on the outside.
Inside is enough CNC equipment to equip a factory!