Hi John,
While "everyone" plays 3C as stayman over 2NT, the most common meaning of 4C over 2NT is "GERBER". One often see things like "Gerber over NT only" written on convention cards. So I suspect 4C asked for ACES, and 4NT showed 3 ACES. While for some players, 4C is "always gerber" most of the world agrees that 1NT-4C or 2NT-4C is Gerber.
I see why north raised to 6S, it seems to him that his partner a) asked for aces, and
![:)](http://www.bridgebase.com/forums/public/style_emoticons/default/cool.gif)
then perhaps tried to "signoff" in spades (not realizing that his partner took 4NT for ACE-asking in response to the 4C ace-asking). But with AKTxx in spades, what else could north do but bid the slam? Besides, it is hard to stay out of slam after gerber or blackwood with only one ace missing.
There are other ways to show this responder hand.... for instance, Kokish transfer to 3H followed by a puppet 3S bid then a bid of 4C. Some also use 3S as a minor suit slam try, others as minor suit stayman.
How would I bid this hand? I think I would bid very similiarly to your action just blast 6NT over 2NT, through Gerber... my auction....
2N-4C
4N-6N
Pass
But it would even be acceptible just to bid 6NT over 2NT. You will not be happy when partner's 20+HCP includes only 1 ace, however. Yo could also do a lot of relaying with 2N-3D-3H-3S-3N-4C showing it, how are you going to find out if slam is making? 31 hcp (minimum) and likely 32, with a running 6 card minor. Make sure you are not off two aces, and bid the slam. Works for me.