Robin,
Well, inverted watermarks would be one clue
Also you could look for trimmed perfs which would characterise booklet
issues.
However, particularly with used, it is more likely that the selvedge would
have been removed,
I believe P.O. thinking at that time did not like additional 'bits' left on
stamps being used on envelopes.
My volume 2 eleventh edition has the pane listed at £7 but I understand that
the latest catalogue rates it at £30 - some leap!
I am sure you will have fun sorting the bag of pieces - just the thing for a
rainy day (such as we are having just now)
Enjoy.
Ron