Here's a question for y'all. Was Einstein aware of the pre-1905 formulas of the form E = K mc^2 where K is a constant?

This question is not as silly as it sounds. First, Einstein was notoriously bad at keeping up with the literature, and second, while working as a patent clerk, he did not have ready access to the literature. According to Pais's biography, Einstein was unaware of Lorentz's work after about 1890 (I'm going from memory so this date is approximate).

So did Einstein develop E = mc^2 in ...ahem... a vacuum?