Some time spent looking through a lot of published photos of GMCs (in U.S. army service) yields the following observations:
WW2:
+ Bridge Plate use restricted to England / Europe
+ Only a minority of GMCs were fitted with bridge plates.
+ Of those fitted with plates, the majority show 9/7, but there are also a few 8, 6 (D Day loading), 9 (Studebaker),10/8 (ACKWX laundry unit), 7 (fleet of shop body trucks), 10/7 (Fuel tanker).
+ Red Ball Express trucks - majority not fitted with plates, all are painted out (were they red or green? No colour photographic evidence found yet).
Post War Europe:
France 10 (and just a few 9)
Norway 8
Belgium 8
European Preservation:
Most fitted with bridge plates!