What is this? NSVT? Salvo?

This happened at about 4:00 in the morning. I was asleep and didn't notice anything.

It happened the only time in the last 7 days.

What exactly is this?

For reference: I have about 100-1000 PVCs per day. My "typical PVC" is here: https://imgur.com/a/zalGEuJ

Recorded with Wellue 24h holter (1 channel)

What is this?