IMPs. 3NT was not alerted.
Quite amazingly, another unusual 3NT occurred at the Allerton Hill bridge club in Leeds last night, and again there was UI. After a strong club and weak jump overcall, EW were soon in a game-forcing auction. South thought North's 3NT bid was natural, when asked, but he seemed to wake up when his partner pulled the double to 4C and found the obvious "sac" at the six level. EW were disappointed they could not beat it. However, West, the club's equivalent of the SB, was not finished. "Why did you pull 3NTx to 4C?", he asked North. "Because I interpreted partner's pass of 3NTx as 'pick a minor'," he responded quickly. Why could he not have ♠Ax ♥J10xx ♦xxxxx ♣AJ, when 3NT is only on the club finesse? "He might have that", said North, "in which case you have psyched a strong club and a game-forcing 2♠, but it does not matter, we have no agreement that pass of 3NTx suggests playing there, nor does anyone else in the civilised world." "The authorised information would be exactly as I stated, 'pick a minor', based on other implicit agreements." "Why do you not think he has made an intelligent call, wanting to play in 3NTx?", continued the SB. "Because only on planet Zog would someone suggest playing in 3NT with a passed hand opposite a weak jump overcall", replied North patiently.
How do you rule? And it does seem remarkable that there is this sudden plethora of "artificial" 3NT bids, but I am sure that my correspondent has not made it up.