Probably a "78, the 79F was 2x5--well the piston kit was & think that relates to engine model number!!. Not that it matters but "79 was only year they stretched the stroke & reduced bore size to 82mm, all other 400's [including this 'trademe' auction i think] are 85mm. bit of useless info there. Cheers.
ps, had one new [79] depressed for the day now!! bloody loved it even if it did break the dogs off 3rd gear straight out of the crate [& down the flat paddock] unheard of but always happens to me, parts were covered as they should have been in this case. Anyway--where was i, dunno.