I don't know the size but you should be able to match it up at the local hardware store.
I bought these discontinued stainless steel CVA cleanout screws from Deer Creek and they will also fit Traditions guns. Notice that they are designed to be turned with a nipple wrench. Did I mention that they're stainless steel? You won't find any of these beauties at the local hardware store. Maybe they'll have one that requires an allen wrench which isn't as handy in the field as carrying a nipple wrench. Or simply give Traditions a call to get the right screw without needing to drive around hoping to find one that fits.
Deer Creek Outfitters, Waldron, Indiana 765-525-6181
Traditions Phone# 860-388-4656