According to Prof. Kabal's site https://web.archive.org/web/20201228133457/http://www-mmsp.ece.mcgill.ca/Documents/AudioFormats/WAVE/WAVE.html , for an extensible wave format the audio format has to be read from the first two bytes of the guid in the subformat.
From this document https://tech.ebu.ch/docs/tech/tech3306v1_0.pdf defining the RF64 format the first field of the Guid is a four byte field (pg 13).
Does this mean that one should treat RF64 and MS wave files differently? One should look at the first two bytes in the former to determine the audio format, and look at the first four bytes in RF64 wave files.
Practically, looking at the first two bytes to determine the audio format should work for both, since the high bytes are going to be zero in R664 wav files.