Hi Lee, as far as i know, the nacelle stayed the same from 1958 to 1962 for the pre units, then had a couple of minor changes after that when it went on the A65 units. I think also the side flashes changed slightly in 1960, if you look at
http://www.draganfly.co.uk/shop/pi-1639534984.htm?categoryId=613 they stock the "top hat and trousers" nacelle assembly (1958-1962) with the flashes and a couple of other little bits as 1958-1959. It seems to be another grey area which someone else may be able to shed a bit of light on.
Hope thats helpful, Richie