In response to my inquiry about my 12 with the successor company of H. Atkin which is in possession of Atkin's records, I received the following:
"It is described as a 12 bore top bar A+D ejector 27" barrels and is No 2 of a pair. Built for Howard Erie and completed on 25/2/1938."

What is meant by "top bar"? Hallowell is silent as to this term. Gil

Perhaps “Top Bar” refers to the bolting system i.e. The term “Westley Richards top-lever bolted ‘doll’s head’ rib extensio” is sometimes applied to some A&D actions

It refers to the top lever as opposed to an underlever or sidelever.

I suppose "bar" has a different meaning when used as "bar in wood" design. Gil

I think you may be missing the punctuation, common problem in old gun records.
I may be meant to read, 12 bore, top, bar, A&D, Ejector, 27" barrels, etc etc
In that case, I think JBG is correct but I am not sure what 'bar' refers to unless the action incorporates an unusual bolting mechanism such as the S Mills top slide but I think that is unlikely given the late date of the gun. Post 1920's actions had become pretty generic.