It is thought by some that the E/R and/or G/R are actually changeovers from steel to Litho, and not actually tests.
Really? How did the current "test note" ranges even become known in the first place? Does anyone have a copy of the original CPMS articles?
I was not aware of any changes in the printing process, especially not that early on in the Beattie-Rasminsky signature. Are you referring to a changeover in the printing of the main design of the notes? I was only aware of the signatures being printed by lithographic process, and not on any $2 as far as I've seen.
I can see how the S/R are completely out of phase with the other prefixes. The plate numbers on the Be-R example I have (>2M) suggest it was printed near the end of the "U" denominational letter. At least that makes it interesting, regardless of what it was used for.
Unless there is some distinguishing feature of the E/R and G/R test notes, I would be a little skeptical about paying such crazy amounts for a note than just happens to be between two numbers in a book.