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.