Mark â€” The pattern number got delightfully screwed up on its way to Pamela's site, as Sowerby allocated them strictly sequentially, so 5927 dates it to between 2050 and 2100! It's actually 2597, a simple transposition error. It's pictured and described by Roger Dodsworth in British Glass between the Wars
, and he found it in Sowerby's 1938 trade catalogue, which is about right for the pattern number, giving a launch date of 1937â€“38. It's probably also in Glen's Sowerby CDs.
I've had a dozen or more of these through my hands in all three sizes, 6", around 8", and 10" (15cm, c. 20cm, 25.5cm), never plinthed. That Sowerby 2506 plinth doesn't even fit very well. So I think a marriage, as you suspected, but it could have been an original retailer marriage for display purposes.
Sowerby called their pink Rosalin
, probably because they had trouble keeping it a true pink. Yours was made on one of their good pink days. 2597 comes in some stunning colours, particularly royal blue, black, and a wonderful vaseline yellow which fluoresces like crazy under a UV lamp. It's also one of Sowerby's best Art Deco patterns.