Malta Spitfire question

[VoyTech]

Graham, are you sure that propeller issues were connected with the Mk VB vs. VC question?
In any case, in general the propellers were not connected to Spitfire variants but to their makers. Those built by Supermarine and Westland were generally fitted with dH props, those from CBAF - with Rotol. The only major exception from this rule is that CBAF Mk Vs from initial deliveries (mid-1942) were fitted with dH props which probably links with the modifications you mentioned.
I'd say the main reason for the switch back from Mk VCs to Mk VBs was that in early 1942 Supermarine's were the only makers of the Mk VC. During late spring/summer 1942 they gradually terminated their Mk V production, moving on to Mk IXs which were needed in quantity on the Channel front. Even those Mk VCs that they continued to build were mostly sent to Rolls-Royce for conversion to Mk IX standard. On the other hand, Westland and CBAF were only just beginning Mk VC production, so for a while there were only Mk VBs available.
The only major advantage the Mk VC had over the Mk VB was that the 'C' wing carried twice the amount of ammunition for each of its cannon than the 'B' wing.