It's just a regular Tower 3a sixpence (S2813). The shape of the crown mark is not important as there were many punches of varying size covering the six silver denominations, not to mention the gold as well.
Sold as a Dovey Furnace 6d? Yet another demonstration of the unfailing ability of people to identify only the rarest possible option.
Despite the Tower 6ds occupying a full page in Spink's tome, there being no plume above the shield on the coin in question and virtually all priced in the low pounds (for this type £50 fine, £200 VF), the vendor remarkably manages to pick out the one priced at £1350 fine and £4500 VF, which although not illustrated, is described as similar to the shilling as is the 4d which is illustrated.
With no example in John Hulett's collection nor Lord Stewartby's, and the number known can be counted on one hand, why anyone would think they have one is beyond me. Brooker had one, but if the vendor had taken the time to look, they might have noticed the bust is the wrong size, the crown mintmark is the wrong shape, there is no plume in front of the bust, there is a plume above the shield, which in any case is a different shape, size and has different garnishing. On the plus side, there IS a VI in the field behind the bust and the legend does start CAROLVS D G, but it all goes tits up at MAG. Never mind, hope springs eternal.
FYI, yours is the sixpence equivalent of the shilling shown below, which similarly exists with mms. bell, crown and tun, i.e. it has the same basic constituents for layout and design features.