It's a bit of a while since the post but let's try to answer. 5712a is a shilling, not a sixpence. What is the diameter? shillings are 26mm and sixpences 21mm. The first weighs about 6 grams, the later 3 grams. The reverse legend looks congested enough for a 6d though, so on that basis I would say S5721/S3620.