The encoding scheme is quite simple, actually, and the answer is that >you can't make a 2 or a 3 from a 1 or a 0 The explanation is as follows. There are 2 pegs at the sides of the >lamp cap, each with 2 edges (top and bottom). Each edge is either cut down >or not. An edge which isn't cut down closes a contact in the lampholder (there are 4 such contacts, of course). An edge which is cut down doesn't. One edge is always present (not cut down). This contact tells the drive that there is a cap fitted. The other 3 edges set the drive ID. One isn't used (it is on the RK07, etc). The other 2 form a binary code, with the contact closed = 1. So >you can change a '1' bit into a '0' bit by filing down the appropriate >edge. But you can't change a '0' into a '1'. And the latter is what you'd >need to do to make a 2 or a 3.