Where did you see this alphanumeric set of digits? Are they stamped into the metal?
If you found this on the top of the driver side rear frame rail behind rear axle, I think you might be missing a digit.
On the top of the rear frame rail there should be a factory plant code, followed by the 6 digits which should be the same as the last 6 digits of the VIN number for the car.
So, in your case, the "B" would be for the Baltimore MD factory, and then there should be 6 numbers. Do the 5 numbers partially match the VIN? Perhaps the 6th digit is too lightly stamped?
Other than that, there were two suppliers for frames for A body pontiacs. I know one of them was Parrish Pressed Steel Co. They put a Part Number on the frame with stencil and white paint. This stencil was on the SIDE of the Driver rear frame rail. Along with the date of manufacture.
Good luck with your resto!