A specialised bid asking for specific aces (not Blackwood). The responses are as follows:
You can see the merit of the Acol 4NT bid from this example hand, which can play in 6♠ in its own right. The Grand Slam depends only on discovering whether partner holds the ♣A. If the response to 4NT is 5♣ or 5♦, bid 6♠; if it is 6♣, bid 7♠ and over 5NT, bid 7NT.