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…

First, go to your local dealer and buy a few correct key blanks.
Remove the seat lock mechanism, and detach the lock housing. Don’t lose the small screw that holds the lock to the striker mechanism.
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.
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:
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.