JW, this is from Brophy's Plans and Specifications of the L.C. Smith Shotgun.
The other ones seem to be oversize, and maybe the gun they were originally from had to drilled out and oversize ones made.
The size is .312 and the threads are 32.


Look on eBay, http://www.ebay.com/itm/L-C-SMITH-f-p-bu...=item43adad7cc2


David