Think you're right Anne.
Just found my original notes on 555817 (a loose leaf in the Blue Book!) where I'd written that this was missing from the Blue Book because it is shown in the Register as Class IV.
Was out early one morning last month and bought another example of this shape - with a Stuart backstamp and showing same Rd. No. - and on the way back to the car stopped to make a final purchase, and knelt to put the bowl and a roll of sandpaper on the ground to free up hands. Made purchase and went back to the car, only realizing at home that I'd left the bowl and sandpaper behind. So would seem a not uncommon shape and produced perhaps for many years - makes me think of a casserole dish. Surprisingly I did get the roll of sandpaper back, eventually.
In fact the cutting on both bowls is quite sharp - perhaps neither were subject to any final acid polishing. Don't know whether there is any significance re this second example having a diamond point No. - might it have been produced prior to 1920 anyone think? - I don't recall seeing other Stuart pieces with Rd. No. applied by this method.