Meegs — Sowerby pattern numbers are sequential. 2610 is certainly a Sowerby pattern number. The earliest Sowerby plinth number I can recall is 2487, which you will find on Pamela's website (http://www.pressglas-pavillon.de). Sorry, I don't know how to provide a direct link, so click on Great Britain and scan down to Sowerby. 2487 is the large pink oval bowl on a four-footed plinth, quite different to later unfooted Sowerby plinths. Click on the index thumbnail for the detailed page. The third image is the plinth number, which is large, serifed and full of character. A second set of numeric punches (different mouldmaker) was in use in later years, apparently mostly for replacement moulds. These numerals are smaller, thinner and unserifed. Many Sowerby plinths are unmarked.
Most Sowerby plinths are numbered 25XX. The latest that spring to mind are the three circular plinths S2614, B2614 and 2616, so your 2610 was produced just before them. I think I've seen both punch fonts used on 2616, so I would be interested in knowing which font is on your 2610.
So, Meegs, certainly one or the other of the Sowerby mouldmakers made your plinth mould, and the plinth was made in Sowerby's works. So it looks as if your set was made by Sowerby. I noticed Pamela's query against her Stölzle suggested attribution. I've not knowingly seen the set before. Please bear in mind other less likely possibilities, such as:
Plinth commissioned from Sowerby by the Continental manufacturer for British (and Commonwealth) sales, orPlinth commissioned from Sowerby by the British agent of the Continental manufacturer for British (and Commonwealth) sales.
I've discounted the possibility of a later mix-and-match as I've never found two identical oval fittings.
Finally, Glen's Sowerby CDs might shed more light on this set.
I hope that helps.
Bernard C. 8)