Marrakai,

The Internet Gun Club is now a subscription only service and it has a good amount of information in its Historical Database. The owners also help in dating and researching specific guns based on the markings on them etc.

Another source of information in the UK would be The Arms and Armour Society.

The Calcutta Manton company still exists but these days they sell only clothing though they still have a license to manufacture guns. Ditto for Rodda and O C Daw, all of whom are in the same locality.

Good hunting!