Thank you - I think mine originally must have had 5 screws, based on (what I think are) original looking holes in the case vs some of the other holes that have been made. My guess is three across the front (like yours), and two at the back (no centre screw).
The frame is badly cracked where the screws should go, so I might go for 8 screws using your sketchup as a guide. It would add a centre screw at the back, and two screws roughly in the centre, into the middle rail. I've ordered a screw set....
Turns out that this Rhodes has spent its whole life from new in the studio from which I bought it, where sadly it has been neglected for the past 20 years.
Thanks again.
Mike.