As those who have followed some of my efforts, I have trying to assemble as fine a collection of late pre-decimal milled British silver as possible. Here is a true rarity, a proof 1944 proof sixpence, sorry these are still in the 2x2:
Are you really sure that this is a proof 1944 sixpence? My Spinks does not list a proof issue for that year, and on the obverse I can see what appear to be two scratches: one by the middle of the forehead and the other halfway between the nose and the bottom of the ear.
Proofs exist from 1926/7 onwards through to the mid 1960s for all denominations and for all(?) years. A few pieces have not come to light yet, but there's no reason to think they don't exist. The non-proof set year coins only exist in small numbers, usually counted on your fingers, though at least one coin was made in greater quantities. The earlier years don't appear to have a normally frosted bust, but the later ones do in most instances. Spink doesn't list all things - there's not enough space unless you wish to carry the book around on a trolley. Proofs can be impaired just like any other coin, so it's quite possible to be a proof with marks/scratches/rim knocks etc.
Those are on the plastic of the 2 x 2. Coin has a semi-cameo effect on reverse but the cameo not required for a proof as Rob said. And, well, I know the series possibly as well as anybody. It is a proof. Also, I bought this from Baldwin as part of a set that included the 3d, this 6d, both shillings, the florin and the halfcrown. And as Rob said Spink - and I assume you mean their catalog as I believe Bull's tome is published by Spink - do not list all these years. Also some are far less rare than others though none of the non-standard years are common.
Not knowing the series that looks every bit a proof just by the way the devices jump off the mirrored field.