Rotol was formed from RR and Bristol as a new source for CSU propellers, and these were entering production and service over the winter of 1939/40 with priority being given to the Hurricane. The Merlin III was designed to take a CSU and you can see the little bulge for this low down on the port nose ring from very early on in Hurricane production (perhaps all but the first 50?). There was some discussion between the Ministry and DH regarding a CS version of the Hamilton design but these were eventually rushed into service in July 1940. I have not seen any reason for the delay but possibly the promise of the superior Hydromatic version was a cause for delay?