My 52 has a metal plate inside the glove box with the VIN, Not sure if they were all like this or not.
Here's where they are on my 51 F1. Driver side stamped in the firewall (this isn't VIN but plant assembly info). Passenger side is a rivited tag on the firewall. Frame stamped. (the location could be slightly foward of where I indicated) Tag on the glovebox lid.
What LowKat said. I have found the frame stamp on the right frame rail as far forward as the front cross member.
Lowkat said to look to the right framerail...my '52 had been cleaned up nicely, chassis painted with a nice black enamel. Look as I did, I couldn't find it... Suddenly, it jumped right out at me, right behind the crossmember, passenger sidetop of frame rail. Oh, and the VIN tag is screwed onto the back side of the glove box door. Screwed, not riveted. Henry did them like this. Thanks, Lowkat.
One of my 48's is stamped on the passenger side and the other 48 F1 is on the driver's side. I believe the frame is supposed to be stamped in 3 places on the same frame rail but I've never looked.