Could it be that the AV conduction time increases gradually throughout the ECG? As the conducted PR increases from 300ms at the start to 420ms at the end, the non-conducted P becomes more visible. Maybe AT + 2:1 block at the upper AVN and a very long Wenckebach sequence developing in the lower AVN.

This is really too difficult for me but I see at least 3 different LBBB QRS morphologies in a repetitive sequence (but with occasional variation) and I can only suggest a type of Wenckebach block in the LBB involving different minor sub-branches of the bundle. Just a guess, though.

I've got a very similar one in my collection!

Could this be 3:2 Wenckebach LBBB?