Thanks for the confirmation on those points.
In my comments I had said that there was no air-trap because the outer surface was not covered, trapping the zigzag design. This seemed relevant because I have heard in the past of moulded (diamond) patterned items being incorrectly described as â€śair-trapâ€ť, even by Bonhamâ€™s auction house!
I can accept that a near-final process for your bowl was the forming of the zigzag outer design in a mould, which would fit with the visual evidence.
Perhaps it is the pull-ups that you think may have had air-trap applied? That could be possible, but I suspect it is not so. My reason for saying this is that I have a pair of vases which do have an air-trap pattern (and it happens to be a zigzag [herring-bone], too). These show a distinct feature linked with an air-trap design. One vase is shown here:http://glassgallery.yobunny.org.uk/displayimage.php?pos=-2389
And the detail is:http://glassgallery.yobunny.org.uk/displayimage.php?pos=-2388
My vases (which, so far, are attributed only as "Stourbridge") are nowhere near as nice as your bowl, but they do show a "shiny" effect of the trapped air at the edges of the pattern. In your piece, there seems to be no "shiny" parts at all, just very nice (and flat) two-tone pulls.
But I would like to see a photo of the base of the bowl if possible. It might make me change my mind.
Also, could you please let me know the page of Hajdamachâ€™s book where you found the reference to a Northwood zigzag pull-up. I know the section on air-trap technique, but cannot find the zigzag reference.
I think it would be a good idea to get an opinion on it from Broadfield House Glass Museum (contact can be made through their website). They ought to be able to comment on the rarity and other points of interest.
I suspect that a value would only be found by contact with a main auction house that handles top-quality glassware. And I suspect itâ€™s likely that the American auction rooms will be the best place since, in my opinion, most of the really good British 19th century glass was shipped out there.