Ford used to hand stamp the last six digits of the VIN on the block of some of their high performance cars. On Boss 302 Mustangs, for example, it was stamped on a flat pad just behind the rear of the aluminum intake manifold. It's very well know among the Boss crowd that because these were hand stamped there were sometimes mistakes. Pretty common to find partial VIN's, or ones were a number is off by one, etc. I've heard of times when a missing digit appeared after a block was out of the car and the owner really performed a "CSI" like process to try to bring a number out!
If the Bronco frame numbers were stamped by hand I'd imagine there are quite a few out there that may be slightly "off".