Just a follow up on this, one spacer did the trick - thanks Clem!
The same problem exists with the SOM 6000 and new Stella 5000 - the spool just clips the base of the rotor. Again, one spacer stops this.
Makes me a touch nervous about what might happen under load. I think some caution is required with using SOM spools designed for Stella 08s with the Stella 13s.