Mine is 6mm, but seem to remember that originally it was also available with 1/4".
These things are ancient history and my memory might be at fault. However, it is relatively easy to check with a piece of known chain. 6 or 8 are easy to tell but 6 and 1/4" may be more difficult.
Ok thanks. It's not for me as I have one with 172 on but a friend asked me for info if it's 6 mm I guess he might need a new gypsy as he's fitting it on a saddler 32. I know A 172 is a 8 mm and the manual doesn't list a 170 which made me wonder if it was an imperial size so the mention of 1/4 inch of 5/16 might be a thought guess he's going to have to take it to his local chain supplier