Derek, the problem with your question is that,
AFAIK, Alan Allbon never stated what changes were being made at the time so, unless someone knows better, we're down to vague impressions and memory.
There aren't many original green head Darts around and I personally don't think that the green head was made after 1951, i.e. Darts only had a green head for about the first year, and perhaps even less time than that.
We were playing with Darts at school in 1952/3, and none of those were green head. I didn't even know that a green head Dart existed until I started collecting.