Watch List notes:

BRITISH COINS, Elizabeth II (1952- ), Brass Trial Threepence, 1955, struck on a light weight dodecagonal flan, young laureate head right, MG incuse on truncation, TRIAL PIECE incuse in field, +ELIZABETH. II. DEI. GRATIA. REGINA. F: D:, raised border rim both sides, rev struck en médaille, crowned portcullis, W G initials below, THREE PENCE legend, 3.40g (Peck -; cf S 4153). Practically as struck, light tone, of the highest rarity.
This trial piece would appear to be about half the thickness of a regular currency 1955 Threepence, highly unusual and the first we have seen.

Estimate: £600-800