Uncharacteristically, in the Hot Rats case this is more or less clear - the master to be used for the new CD reissue is the one that was prepared for the Classic Records Vynil reissue from 2009: http://classicrecords.com/item.cfm?item=RS%206356-150G . I have not heard it, but the reviews are very positive, and it is apparently indeed very close to the original 1969 LP mix and sound.