Hi Clive, I have an English, serial no 575, that I am (slowly) restoring which has the same slots cut into the action box. I had never seen that feature before. So far I have been unable to identify a maker - my first thought was Lachenal, but the action is not typical. I'd be interested to see what you find out.
incidentally, my English also has a name inscribed: Amelia Tidd. 1875.