Dear Roundsworth;
Thank you very much for the information of your most interesting find. I have looked for a long time to find something other than a boxlock that Arthur Howell made (or in your case partially made) and your find has mad my day.
Your description caused me to believe that it is a gun that Arthur Howell re-barreled at his Weaman Street shop in Birmingham.
I am going to send you a PM with my email address and ask that you allow me to contact you via email and that you send me some photos of this gun.
Kindest Regards;
Stephen Howell
Blythewood, South Carolina