All QARANC (Queen Alexandra's Royal Army Nursing Corps) staff wore belts to identify thier trades within the corp. Registered nurses (RGN, RMN) would wear a scarlet belt, the enrolled nurses (ENG, ENM) wore scarlet and grey, the Health Care Assistants (HCA) who were previously called Ward Stewardesses would wear all grey and the dental nurses and dental hygienists would wear green with a grey stripe. Each QA would wear the standard QARANC Corps buckle.
The mystery belt looks confusing. I suspect that it is a QARANC belt, but with a shop bought 'nurses buckle', possibly in hallmarked silver? They can be purchased very easily and cheaply for such a plain buckle. The adjusters would have been purchased with the clasp as they are of the correct proportions.