1)Auto advance. OK thank you, that's good to hear.
2)Primary rattle noise. No was happening at idle. Took the round inspection screwcap off and the top run of the primary chain had perhaps 3/4" of play. Settled down and was quiet after a ride.
3)Oh OK. Again, good to hear. As you say, brakes may not be set up correctly. Rear certainly had too much travel.
4)Magneto kill button. yes looks like I should fit one of those.
Bike was very pretty. In black and chrome. Went nicely once in motion.
1) That's normal - should have auto advance-retard fitted
2) No idea, unless it's happening under load in which case it's the engine shock-absorber cam slipping (unfortunately, my particular area of hard-won expertise for these)
3) Half-width is standard spec for '55 and often considered better than the later full-width variety, they are often not properly adjusted (same hub/brake as singles and 500 twin, see a couple of other recent posts).
4) Should have a magneto 'kill' button.
(Engine-wise, it's the same as a BSA A10)