I recently finished going through the process of reviving a 1973 R75/5. One of the challenges I faced was that of missing keys. Sure, the ignition nail key was there, but the keys for the seat lock and the steering head lock were missing. I needed to remove the lock for the fork in order to complete frame painting, and really wanted matching keys for both the seat and fork locks if at all possible. This precluded me from simply drilling it out. I was pretty sure that the locks were original to the frame, and that the keys should be the same for each.

So, here is what worked for me…

1. First, go to your local dealer and buy a few correct key blanks.
2. Remove the seat lock mechanism, and detach the lock housing. Don’t lose the small screw that holds the lock to the striker mechanism.
3. Take the blanks to a competent locksmith and ask them if they can make an impression cut key. Keep looking for someone who will give it a try. Now, just so you know, I was advised that they might damage the lock mechanism while making a key in this manner. If they (or you) are uncomfortable making an impression-cut key, the lock may be disassembled and the tumblers opened up for their key making efforts, but it is critical that they get the pins back together in the same order to have the fork lock work with this key.
4. OK, we’ll assume that you have a key that works with the seat lock cylinder. You will try it in the fork lock and, surprise, it won’t work!! Here’s what to do:
5. Look at the key that you have. You should see four valleys in the key blank, which were cut by the locksmith. You will also notice that they are evenly spaced along the key, and also that there is room on the end closest to the handle of the key for another cut to be made. Starting at the end of the key furthest from the handle, we’ll refer to these as cuts #1, #2, #3 and #4. The trick is that the seat lock uses four tumblers, and the fork lock uses five. You will need to cut the last one, #5, yourself.
6. Take one of your working seat lock keys, and a small appropriate file (one that can cut a “V” in the key blank the same size that the locksmith did) out to your bike and prepare to do a little locksmithing. The valleys for these keys are cut to four different depths along the key. You will want to begin to file the key at the spot where the fifth cut would be if the locksmith had cut it for you. File a little, check it, file a little more, and check it again, and so on until you get the fork lock to work. Take your time, and compare the cut you are making to the others on the key for shape, etc.
7. Once you get one good key, the rest are pretty easy. If you get all the way down to the edge of the indent in the key, you’ve gone too far. You either went too fast (hard to do) or the fork lock is not keyed the same as the seat lock on the first four cuts. If this is the case you will have to drill out and replace the fork lock if you want it to work, or if you need to remove it for any other reason.

Good Luck! I kept one original key and made a few others off of it as a guide, and have matching keys to go with the newly freshened-up bike.