Difference Between Booklets?

I have a couple of panes of Scott #807 3c Jefferson that appear to be from a booklet. However when I researched the books I have the only information I could find was a listing for either a booklet av or a booklet a. There were four booklets; 2 of 37c and 2 of 73c; the 37c being I am guessing only 2 panes a stamps and the 73c variety being 4 panes. What does the the av or a stand for please; none of my books explain that. Thank you.

This is just a guess, but ... booklet pane 807a comes with two different sized gutters. The 3mm gutter was issued in 1939 ... the 2.5mm gutter was issued in 1942. The gutters are vertical ... it is the distance between successive rows of stamps. The earlier 3mm gutter is the more valuable of the two. The v is probably added to the 807a to indicate that it is a different "variety".