Why is the word Britannia (with a single "t") abbreviated as "BRITT" (with 2 "t's) on some old British coins?

Question

Why is the word Britannia (with a single "t") abbreviated as "BRITT" (with 2 "t's) on some old British coins?

Answer 1

It is a way of pluralizing in abbreviations. the "Brit" is just for Britannia (Britain) but "Britt" is for Britanniarum meaning the British territories. Yes not very logical as Britanniarum only has one t but that is how it is done.  

Answer 2

Britt stands for Brit Eklund (Peter Sellers's widow) and May Britt (just a guess)

Answer 3

It follows the old practice of doubling the last letter of an abbreviation in order to signify a plural. You may still see this practice in the abbreviation MS for manuscript, where the plural is MSS (the second S here is a doubling, not a plural 's'), and likewise TSS for typescripts.

Although it may seem a useless custom, consider the plural of the abbreviation of Mister (Mr), if you were addressing several people, where you would write, say, "Dear MM Smith and Jones", instead of "Dear Mrs Smith and Jones". (Alternatively, you could of course use the French form "Messrs".)