Spot the Difference - law 46
There are two Js in Dummy that can legally be played.
Declarer says "I'll have the jack". Law 46B3 applies.
Decarer says "I'll have the jack of..." and then pauses.
Does he still have to play a jack?
What if he continues "no wait, I'll have the Queen of Diamonds"?
What if he claims he was calling for the J of diamonds then realised there is no DJ in Dummy?
Does any of this change if there's only one Jack in Dummy that can legally be played?
As an aside, Law 46 talks about "except when declarer’s different intention is incontrovertible". That sounds quite a high bar to me, but how high is it really? What does "incontrovertible" consist of? It must be much stronger than the test we apply in Law 25A: Unintended Call, where we tend to believe the player.
Comments
It crosses my mind that a quick-thinking declarer could complete "I'll have the Jack of…" with a suit that doesn't contain a Jack, rather than trailing off, in order to unambiguously be able to change to a different card (Law 46B4). (This has also made me wonder about an example that isn't on your list: "I'll have the Jack of Notrumps".) That said, doing that sort of thing would probably violate Law 72B1 (although it might be hard to prove).
I suspect this comes down to "when is the declarer considered to have finished designating a card?" which is a topic that doesn't seem to be covered by the Laws (but the Laws likewise don't cover other similar concepts, such as when a bid is considered to have finished being made – the EBU has a regulation covering that but there isn't a law about it, unless you count Law 80B2e which is the Law that gives the EBU the right to make the regulation). Law 80B2e says "bidding and play", so it seems as though regulating authorities would be able to make a decision about this too. I don't think we currently have a relevant regulation, though.
A similar concept is "when has the declarer finished making a claim?", which IIRC we're normally fairly lenient on.
I think, legally, I'll have the Jack is a completed call for a card. So the law on what it means would apply. Whereas "Jack of" is an unfinished statement that implies some intent to finish it.