I talked to a guy tonight and he exclaimed that the VIN is on the metal (didn't say frame but I assume that's what he ment) near the steering column. He also stated that, "You got to pretty dern much take the entire thing apart to get to it." On the '41s the VIN is just in front of the cowl on the driver's side right next to the steering gear on top of the frame, which may be the same place for the '28.
I said in the email that the book said that the VIN was on the engine just above the water inlet, but you may be careful about that since the engine may have been swapped out. Tell us if you find it, good luck!