very cool!
(I have some VERY old 18x24 glass negatives from Ilford... Anyone know how old?)
Did your plates still have their instruction page enclosed? There
should be something like C.37 or whatever down in the bottom corner. The letter of the alphabet is indicative of the month A=January, B=February, C=March, etc., and 37 would be shorthand for the year 1937.
Maurice Fisher's site "Photographic Memorabilia", mentioned in the article Simon linked to, has a listing of
Ilford's History and Chronology here and, just beneath the introduction there are examples of the various logos used by Ilford on their products. I couldn't make out enough details in the image you've posted - check the flag, the cloud formation and how the sky is shaded to see if you can find a match amongst those shown.
Hope that helps! I've been in contact with Maurice in the past and made a great deal of use of his site as a point of reference and he's extraordinarily helpful and a lovely human being.
