In my experience they are rare on British guns, but not unheard of by any means. I own the Westley falling block on the cover of Winfer's book and it is 3 pieces soldered on and very well fit. A customer has a lovely Jeffery PD Farq with an integral rib. It is pictured here.

http://forums.nitroexpress.com/showflat.php?Cat=0&Number=188952&an=0&page=0#Post188952


Firearms imports, consignments


[Linked Image from i.imgur.com]