Dear Rob,
The thing I learned at the Conservatoire with 65slashed as first non root dominant chord meant, for a e, that the notes would be e g bflat c (dominant of f). Hence 5slashed would be a diminished 5th. I know nothing about slashes that augment the interval, but again, it’s really NOT my field!
I’m just wondering whether there’s something specific to french music when it comes to figuring basses.