Britian is about the only source for sleeving. the 13/1 is a diameter like the 0.736" measurement. Seems as it passed the 1st series of proof in the Birmingham in the 1904-1925 period. Then sleeving and subsequent reproof in the 1954-1989 period. From time to time I shoot non-tox thru somewhat open chokes in sleeved hammerguns. Does resemble an export or colony sporting longarm.

Kind Regards,

Raimey
rse