The Baker patent '12/20', as it is often described, is not a 'self-opener' in the true sense as the opening force is only significant when one or both barrels remain unfired, similar to the C&H and normal Boss SLE's since the early 1900's. It cannot be compared with the Purdey or H&H SO (and very rare Boss SO's) mechanisms which provide the same SO force regardless of the state of the locks.
As described above, the mechanism only cocks the main springs when the gun is closed: they remain only lightly compressed when the gun is open or disassembled and this is arguably the advantage of the mechanism.
However, the '12/20' monica was coined by Burrand as they were built by several makers as a '12b with the weight of a 20b'. Others used the inherent strength of the action and backaction locks to build a super strong 12b with the weight of a normal one. This was the case with the Blanch example that I shot for several season.
In my experience, the major drawback of the gun was the significant effort needed to close the gun in a hurry when walking-up or rough shooting: for driven game it was lovely.
