From Peter Maunder, "I agree, this looks like another Tiverton seal. It is a two-disc seal (rather than a four-disc seal which was favoured in Exeter) so it is probably from Tiverton, and as you say the number is in a circle. I also agree that the number is 21 1/2. There are two scallop shells, and also some dots or pellets. John Upcott and William Lewis both used two scallops on their seals (figures 19 and 25 in the catalogue) but this seal is a new type.
My guess is that it dates to the early eighteenth century and does come from Tiverton."