What's the chemical formula of "Everitt's salt"?

According to my school textbook (which is notorious, for being in possession of several errors), a certain "Everitt's salt" is a complex that results from the reduction of Prussian Blue.


Everitt's salt (according to our inglorious textbook) has the chemical formula $\ce{K2[Fe(CN)6]}$... which I straightaway dismissed as nonsense, because it would mean it's got $\ce{Fe^{4+}}$ (ferryl ion). From my (very limited) knowledge of the subject, the ferryl ion doesn't look like something that would occur in any reasonably stable compound; since Everitt's salt is, well, a "salt" (and since salts can usually be crystallized into stable solids).


Google wasn't much help in this regard,



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> [![enter image description here](https://i.stack.imgur.com/novXN.png)](https://i.stack.imgur.com/novXN.png)
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> I've no idea how to interpret the formula that is supposedly that of Everitt's salt 
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Questions:


1) Is there really something called "Everitt's salt"? Is it produced by the reduction of Prussian Blue?


2) What's its chemical formula?




According to various sources Everitt's Salt has the formula $\ce{K2Fe[Fe(CN)6]}$, where both $\ce{Fe}$ are in the $+2$ Oxidation State. Everitt described it as a yellow precipitate, resulting from the reaction:


$$\ce{2 K4Fe(CN)6(aq) + 6 H2SO4(aq) \to 6 HCN(g) + 6 KHSO4(aq) + K2Fe[Fe(CN)6](s)}$$


Acc. Everitt, the wet salt can be oxidised to Prussian Blue by the action of air (oxygen).


[Source](https://www.researchgate.net/publication/281176633_On_the_History_of_Prussian_Blue_Thomas_Everitt_1805-1845_and_Everitts_Salt).


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