Hi nick not an easy piece to visualize how it sits/stands - the factory image makes the perspective difficult to understand.
I could be wrong, but it's possible the factory pattern might read 'Sweet 2215' - a number which would fit generally with others patterns on the same page - but that's really only a partly educated guess - Fred may possibly know the answer to this one.
Glen Thistlewood did provide an explanation for the missing factory pattern nos. some while back, and this situation occurs on other drawings from the catalogue, which regrettably is unavoidable.
The only source I know of where both Sowerby factory pattern nos. AND Board of Trade Registration nos. are shown, is in Simon Cottle's booklet issued for the Tyne & Wear Museums Service ' . Sowerby Gateshead Glass'.
Unfortunately, Simon Cottle's list of factory pattern nos. is very lean after about 1880, although he continues to show Registration nos. up until 1894 (Rd. No. 238352) - which may well have been the final Registration under the name of Sowerby & Co. - after which the company name becomes George Sowerby - and it looks as though Registrations re-commence in 1900.
Although I haven't checked, I would have thought this sort of information may well be on the Thistlewood discs.
Anyway, it seems not possible to link factory pattern 2215 (if that is correct) with a corresponding Board of Trade Rd. No.
The shape of your 'sweet' appears unique, and it may have been Registered, but always the possibility that it never was.
Probably better not to refer to the numbers on the Sowerby catalogue pages as 'design Nos. - my reason for suggesting this is that most people know the catalogue details as factory pattern Nos., and reserve the expression 'design Nos.' for the the Board of Trade Registrations.

Let's hope Fred can give us the answer to this one.