I am going to make an educated guess and assume he used it on other projects. I also read Mick's book and he made it clear that he made his living from session work which paid him far more money than Japan ever did. And IIRC his Klein went missing in 1992 though RTC finished recording in early 1990 and I don't believe they ever played live.
So if he had the Wal and another 5er odds are they were for the odd call when low B was needed.
Again just a guess but it makes sense. I only wish Mick were still among us to set the record straight!
It is hard to tell for sure cause he also does a lot of detuning of his E string of his 4 string. Even to B sometimes.
