Thanks Beth. There is possible confusion or doubling up with TBF-QM-51 and TBF-QM-62. From a quick check only it seems the City gift was of pearls and diamonds, not solely diamonds. Do we know if any of those chokers are solely diamonds. I often find it so hard to distinguish between them.
Edit to clarify: it is likely that Boffer has documented the one piece twice under separate entries. At this stage it seems to me that the issue hangs on whether Queen Mary had two collars of that style - one of all diamonds and another of diamonds and pearls. It does appear to me, from Beth's extract from "The Penny Illustrated" that the City gift was of diamonds and pearls. Edited again to fix the refs.