As far as I know, there should be a number stamped into the neck. Any evidence it's been ground off? It might also have been filled in with bondo when a PO "smoothed" things out: this happened to me once with an old BSA. I got pulled over, and at some point the cop says "The frame serial number is missing", and walks back to the car and gets on the radio. I'd never thought to even look for a number, but now I was scared my bike had been stolen, and I might even be blamed as the thief! I frantically scraped away the paint and bondo on the neck, and uncovered the number. Whew!