Have you visited www.graflex.org? If you haven't, you should.
There's a mythical non-existent Graflex serial number book. The latest 5x7 Speed Graphic serial numbers I could find in it were assigned 8/1/1940. It appears that very few 5x7 Speeds were made. The reason has to be very weak demand.
War-time production was entirely for the War Department, which seems to have bought no press cameras or Graflex SLRs larger than 4x5.