I would imagine a Belgian conversion used the same bayonet as one in original flintlock. If so Hardin's "The American Bayonet" says the "blade is 16 1/8 inches long, 15/16 inch wide, and 7/8 inch thick...the shank is 1 7/32 inches long...the socket is 3 inches long with a .813 inch diameter bore."

The H&P conversion did use a different bayonet, although I believe it is interchangeable with a standard M1816.

JHG