Pauses. Mechanism?
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epfellow 
Two that come to mind:
1. SA nodal block. I considered Type I, but the P-P doesn’t exhibit any shortening. The P-P at the lead swap, when compared to the rest makes SA nodal block seem less likely.
2. Concealed non-conducted junctional premature beats.
Sinus arrhythmia?
I’m calling it type I SA exit block. The first P-P in each group is longer than the subsequent P-P intervals, and each “sinus pause” is less than 2x the preceding P-P interval.
@Christopher, we should still be seeing atrial activity if this was due to concealed junctional extrasystole; that’s a cause of dropped P’s and pseduo-AV-block but not isolated pseduo-SA-block.
@VinceD, I didn’t count the P-P as being longer, but as you deftly point out there is no evidence of any P’s to explain my junctional complexes. I’ve measured out from inside the T-waves and the SA node would have recovered before the next P-wave arrives.
Busting out Paint, we’ve got a 3 pixel longer P-P! So I think you’re absolutely correct. All of the footprints of Wenckebach are present.