My cutter looks just like the one in the Bev Fitchett link... it is a piece of file with the faces ground off leaving the edge as a multi tooth cutter. Easy to make but not as effective as i would have thought considering the source. Oh well!
To be honest there is an idea I hadn't thought of is a circular saw blade. If the carbide is narrow enough and not too long it might work and if so the carbide wouldn't likely dull for a whole choke tube or maybe even a whole barrel. Obviously cutting speeds are low when cutting by hand.
For now I think I am stuck with either replacing the cutter I have in kind or possibly making a hook or scraper style cutter out of a file or high speed steel as Randy suggested. Since I don't have easy access to tool steels, unless possibly I get something used from a local shop and grind it to shape, I'll likely use a file and carefully grind a cutter out of it. As I said, I am at least 1/3 the way there so at worst I'll have to make a third cutter to get the full depth of rifling. They really aren't that hard to make but I do have to make sure they are pretty much the same length and for sure the same width to fit the partially cut grooves.
If I do this again (IF!) I'll look at the circular saw blade Idea. Or possibly another source of carbide cutter. Good idea!
Longbow