OK, it's a Huang Frontiers Harp (sic) and it is a plastic 48hole 2-and-a-bit octave, and it has been to my ear consistently on-key since I bought it. It cost about as much as 10 cups of cafe coffee.

Steve, you are right, they are not reed pans, but plates. They are brass with brass reeds which are noticeably individually tuned by grinding, and fixed to the fluted body(?) with about 10 really tiny cheese-head machine screws.
The nomenclature is because I am a concertina-ist (sort of) and that is what they are called therein.

After my treatment there is just one bass reed not working properly. There were no noticeable deposits in there, just the unshiftable black surface corrosion.
Chris.