This picture is a RBL Launch 20g barrel assembly prior to brazing the lump, if my memory is correct.
While we're on the subject, I've never seen any evidence from actual use in service, that any one of the common methods of joining the barrels was inadequate.
As for the quality of the gun dictating the method of joining barrels, I recall an episode of Shotgun Journal where Bruce was at A&S Famars and they showed an o/u being built with a monoblock. Frankly, I would not put barrel joining method on the list of things I look for in a decision to purchase a gun.
As for heat affecting the barrels, Tin/Lead solder has a eutectic point of about 360F and shouldn't make any difference on barrel steel. Brazing will be much, much higher temp and will anneal the barrels in the heat affected zone.
This link to Hallowell's page shows various methods of joining.
http://www.hallowellco.com/lumps.htm
