Just for the record, bronze guides were only used in the alloy heads until some time during 1957. Iron guides were used thereafter and were also fitted to the early A50/65.
Whether it was to save money by using the same guides in all heads or some other reason, I guess we will never know.
I have iron guides in my alloy heads with no issues, except for the '54 head which needs new guides.
According to the parts book, the change was from CA7SS 4025 & CA10R 5603.
Trev.