That you and the Copper Elf have visited Sheffield foundries and know a little about them and their products is precisely why I'm going to mine y'all for all the information I can get. I have no primary sources other than the two colonial anvils I own and one of them doesn't have any discernible marks. Of course I have Anvils in America but that is only a starting point and further investigation is always warranted. Postman refers to your 5th foot by both terms, i.e. foot and toe (see page 14). If he designated the S a makers mark I missed it. He thought it meant Sheffield (AIA, pg. 16), hence my question about how broadly it was used. Anvilfire.com says this, "The S anvils have several unique features that mark them as coming from the same British manufacturer. Face slope, rectangular handling holes and the bold fifth foot." Anvilfire doesn't name the company, but if that manufacturer is Alsop, so be it. I have not seen a sufficiently large sample of these wonderful anvils to draw a reliable conclusion and defer to y'all.