[Nick Holliday]

There is an official drawing of a timber Lancashire & Yorkshire signal box with the horizontal sliding sashes in "A Pictorial Record of LMS Architecture" by Anderson and Fox from OPC which may be of interest.

 

[Nick Holliday]

I don't know if you are still looking, but by chance I came across Modellers' BackTrack Volume 4 No 6 (February 1995) which has an article by T T Sutcliffe on "Standard LYR Signal Boxes". There are several photos, two 7mm drawings of size "5" and size "6" boxes, and an analysis of the various sizes of box, with lists of examples. All in all it looks pretty useful, if you can lay your hands on a copy.