[Davew]

Don't forget that the M61 used a carburator like the one on the Mk5, with anti-overfloat and not a full G capable like the one on the M66.

It did use one like the MkV, but it still allowed negative 'g' manoeuvres. A diaphragm across the float chambers was fitted as standard from march 1941 allowing negative 'g'.
Following that, a modified version of the SU carburettor was fitted from 1942 to single stage and two stage merlins - the anit-g version. And then in 1943 the Bendix-Stromburg carburettor was fitted which basically provided direct injection.

I cannot find specific dates for the introduction of each modification though, so the very early IXs may have had either the diaphragm or the modified SU carburettor. Either way, all versions of the MKIX were capable of negative 'g' without the engine cutting out. At least as far as I am aware...