By the time the MkIX was introduced, the radio and IFF fit had been up-dated for some time.
The radio was VHF, so the mast should just have the two prongs, where the triangular mount for the 'old' aerial wire attached - but no wire.
Likewise, the IFF aerial wires were dispensed with, and a dipole rod aerial fitted beneath the starboard wing. I haven't got a pic immediately to hand, to show the location, but here's a shot of the rod antenna on my 1/32nd scale Spit Mk.XIV conversion, which might help.