As mentioned in the last article, the GT750 tank filler cover lock as used on the 1976 and 1977 GT750 is a different animal. Unlike the other locks used by Suzuki, this one has a crimped bezel which retains the lock tumbler, making it a real pain to deal with. For this reason, a number of people (including myself) will suggest that you try to find a tank cover lock complete with key because it is then easier to make everything else match it, rather than the other way around.
Referring to the photo below (just click for a large image), notice that the lock tumbler itself actually only has one leaf. This means a range of different keys will open it, as many keys will have the same lift at that position on the key. I have three locks in my spares box, and one can be opened with a 140 and a 150 key, and another will open with the 207 or 150 key. In theory, if a person had enough different keys then you should be able to find several that will fit any given lock. I suppose at one time, there would have been markings on the barrels which would allow you to work out which range of key numbers fit which locks, however only one of mine has anything interesting stamped on the barrel and the others are blank. There are what appear to be different casting numbers on the end of the barrels, but these numbers do not appear to relate to which key to use. So if you are in the unfortunate position of having a tank cover lock, but no key then you are in a bit of a pickle.
The good news is that Suzuki still sells these locks under part number 44280-37010 for about $34 in USD in the USA or € 35 in most of Europe. For some reason they are seriously silly money at the one place I checked in the UK (£62 !!! 
). As a matter of interest, the same lock is used on several models of 1977 through 1979 GS750, GS550, GS400, GS425 as well as the 1976 RE5. Given that the new lock comes with two new keys, a new retaining clip, new gasket and also given how easy it is to re-key the other three locks on a 1976 or 1977 GT750, this then really is the logical path to take if you want a matching set of keyed-alike locks for your restoration.
Of course, if you are a purist, then go ahead and re-key the one you have ! The only hard part is first removing and then re-fitting the bezel without damaging it. You can make your life simpler when re-installing the bezel and either saw tooth the inside edge of the bezel so that you have many small triangular tabs to fold over (the same trick also works on the instrument stainless steel retainer rings by the way) or cut excess material away with a Dremel to leave four tabs (as seen in the photo) which will make it much easier to re-crimp into place. The bezel, the tumbler cover and the trap door are made of stainless steel and so will clean up nicely with a bit of care.
Good luck ! 