Yup, you're spopt on with your information, Mal. It indeed had a military beginning with the Baker and Brunswick rifles. Greener talks a little about it, claiming it was his invention all along. Unsworth devotes a few pages of top-notch information in his book THE EARLY PURDEYS.
Here's another, related question: Besides the belted balls, there were also "winged" bullets, conicals meant to do the same thing only better. If one were to give a mould maker a smaple of one of these (or any other projectile), could he make a mould from that? Kind of an inverse property type of thing(?)