The Stovebolt Site Forums The Parking Lot '60-'66 Trucks What are these numbers on the frame?

I was cleaning up under the cab before I weld in a new floor pan and found some numbers stamped on the frame. They are on the drivers side, right under and slightly behind the front cab mount. Its not the VIN, anybody know what they are and what they mean?

Here's the numbers:

3881391 120 5 19 113

Thanks