Thanks – this has confirmed what I thought
I went to a web site http://www.sproketsunlimited.com to get chain size info and they use three dimensions to define the chain – pitch, width and roller diameter.
I was quoting the width as 0.305″ but did not give the roller diameter. For a 1/2″ pitch and 0.305″ width the roller size is 0.335″ i.e. as you say. Hence I believe we are talking about the same size i.e. 1/2″ pitch by 0.305″ with 0.335″ rollers.
Also, we are talking about the same length – a length of chain with 69 links which needs a split connector to make a total of 70 links in the loop. This corresponds to 70 rollers in the chain length (the web site uses this definition for length).
Having had to fit the primary chain before, I will certainly use the old one to pull it through as I found it really problematic to get a length of wire to work. I also expect to have to remove the oil tank to get enough space which is another awkward job on a Brum