The correct Treso part number is 11-50-01; the ToTW catalog number for that is RST-A.
Looks to me like the ToTW pages says RST-WA is "Nipple, 1/4-28 thread, for 1847 Walker revolver, Ampco bronze".
It also says that RST-A is "Nipple, 1/4-28 thread, for standard rifle, #11 cap, Ampco bronze".
FWIW, I bought my Tresos from the Possible Shop; I bought the ones they indicate for Walkers and Dragoons (11-50-136) and they work. I just pulled one out and it measured 0.636 inches.