Your are correct regarding the proof marks

[Linked Image from photos.smugmug.com]

'13B' is the bore measurement. Fractions were not used until after 1887 so the bore could be .710" to .728". '13' is not at all uncommon since it could represent a just slightly undersized 12g bore, standard being .729".
'14M' is the muzzle measurement of .693" to .709".
So the bore and choke need to be measured.

Lots of information regarding the maker here
https://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=339909
and
https://doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=503991

https://sites.google.com/a/damascusknowledge.com/www/home