Indeed Mike. As Andy says, an association need not exist to have a valid contact with any DXCC, and for the person you worked to register and claim the QSO as a chaser.
OTOH, it certainly does need to exist in order to work an activator in that DXCC, which is why international S2S contacts are rather good fun. I’m pretty sure I had the 2nd ever GW to HB S2S (because G3CWI who was joint activating with me at the time - Arenig Fawr GW/NW-011 - got the first IIRC).
I may have the first GI to G S2S (Agnew’s Hill GI/AH-005 to Black Combe G/LD-030) with Alan M1EYO in 2003, but I’m not sure.
Perhaps Andy can churn out a grid with all the associations, with the date, summit refs and activators entered into each cell corresponding to the first S2S QSO between those two associations. A very quick and easy job I would have thought…