As Don said, a muzzle loader powder tube would work. You will need one that measures
.499 / .500. A little bigger can be brought down by wrapping emery paper around it, but I wouldn't want to take much off.
You do know that Belding & Mull are back in business and that charge tubes are available ? They are a little pricey though.
Call up Buffalo Arms and ask them if they would mic the o.d. of this one
http://www.buffaloarms.com/Black_Pow....aspx?CAT=4785
In fact, I might just do it myself, my friend needs one and I could use a spare.
Where in S.W. Ohio ? I live just outside Akron....
Also as Don said, the ring with the screw is for locking the plunger. Since the brass tube is a bit thin to try and tap, the ring is steel and has the threads in it. There is a clearance hole drill in the brass tube. The locking screw is threaded into the ring and goes through the clearance hole and presses up against the plunger. The ring is a pretty good fit on the brass tube so it doesn't slop around.
How new is new ? If it is a bit tarnished, a little bit of Mother's Mag Wheel polish and a paper towel and some elbow grease will shine it up !
This is the one I redid for Smitty Pratt in New Hampshire.