Result 4♠+1 +650NS
2♥:'Muiderberg' 5 card hearts with at least 4 cards in either minor.
3♣: West hesitates but explains to South that he assumes it to be convertible : pass with clubs, or correct to diamonds. East would typically hold at least 4-4 in the minors for that call.
The TD is called after play. NS feel damaged: a double of 3♣ would have been for penalties, but South refrained from that because he forsaw EW running to 3♦ (West confirms that he would have bid 3♦). Had South known that 3♣ was real he would have doubled for penalties as EW now for certain have no diamond fit. The convention card mentions that 3♣ indeed is convertible, but that is when the opponents pass or double. EW agree after some discussion that 3♣ just showed clubs and nothing else.
How do you rule?