I agree, however I am not sure if renaming to Radix-64 is more clear. It seems that this term is also used by some conversion algorithm. My proposal for the documentation is therefore:
- mention that the conversion algorithm does not follow the Base64 encoding from RFC4648
- provide some information how you can make use of the Base64 encoding (in case you do not want to deal with conversion yourself).
We may also should highlight the fact that the GUID is 128-bit number where you cannot add missing bits/bytes to the end.
The revised version from Moult sounds good and hopefully helps to avoid missinterpretation.