[kgr]

He had following bidding at club match:
Opps bid:
1♦-1♥
1♠-2♣
2♦-4NT
5♦-6NT
2♣: 4th suit forcing
6NT is doubled by your partner. You play Lightner doubles.
What lead is he asking?
Would it make a difference if LHO did bid 3♦ iso 2♦ (rest of bidding the same)?

I wonder if the DBL asks ♦ (most unusual) or ♠ (less unusual, and so more likely to happen, certainly if LHO did bid 3♦ iso 2♦)

(This happened at MP's and DBL was pure penalty with 2 Aces - so I will not show hands - but suppose this happens at IMP's).

[Free]

On one hand, you can say it asks for a ♦, which is dummy's first bid suit. However, partner had the opportunity to double 5♦ which he didn't do. With ♦AQ this may still be the case, but it's imo not necessary to double 6NT on such holding. Which makes my final conclusion that it should ask for a ♠ lead.

[kenberg]

My understanding is that the double of a suit slam is often on a void and so asks for judgment, but the double of a NT slam asks for the first suit bid by dummy, pretty much w/o exception (well, maybe if dummy started with an artificial bid such as a strong club it asks for the first natural suit, something to get straight w pard).

I don't see this as an exception. It's true that pard could have doubled 5D but if he holds, say, AQ tight he may not welcome a redouble. I'd say any exception needs to be discussed and written down, else it's not an exception.

[Tola18]

But. Diamonds are dummys long suit. Declarer must probably play it herself as it is NT. If the setting is in diamonds, it will probably be done anyway.

Diamonds out would be more natural if it was a color-slam, say in clubs.

[whereagles]

Easier if I had a hand to look at. In abstraction, it's more of a guess, though there's a tendency towards diamonds (e.g. the aforementioned AQ). After all, declarer might be counting on 6-7 clubs from his own hand to take the 12 tricks.