'F' is the grade of your hammer gun (not Field), and it was made about 1905. A Research Letter would confirm, and possibly the dealer that ordered the gun. 'BH' could be Belknap Hardware in Louisville.
'BD' is likely the Liege tube maker Bauduin Doyen
4 over 6 is the pre-fit & finish barrel weight of 4 lbs. 6 oz.