Had a long chat with David, who has an excellent memory, and he mentioned that there must have been a transition period between the early block and later block, with the hybrid type coming out for a short while.
He still visits the core shop foreman who lives locally, and it was this man apparently that suggested making the change to the casting.
So as far as I can see, the story is only half-true, but interesting nonetheless.
The article I referred to is in Stationary Engine magazine for May 1998, part of David Edgington's monthly "Engine Torgue" feature.
Peter