Great to see a Mk.21 in the GB, and I wish there was a full, injection-moulded kit in 1/48th scale.
However, as far as I'm aware, there was only one Mk.21 converted to trial a contra-rotating prop, Serial No. LA218.
I presume that the extra prop blades in the kit are for the Seafire variant, given that there's also an alternative "hooked" tail assembly shown.