They are 4-pin mini-xlr on my headphones. The Q701 had a 3 pin variant originally. Most headphones wire the ground wires from both drivers together. 4 pins instead keeps them separate. I can then use different cables to either keep them 4 pins or merge them again into three pin.

The reason for this is I have certain amps that only work with them being separated.

Little bit of dremeling and most headphones can be changed.

Stock K550’s didn’t have a detachable cord. Later variants did. I usually recable them so I can use them with all my amps.

Not the prettiest cause my hand slipped and made a gouge and time has scuffed it from plugging/unplugging but it’s been fine for 10+ years.

I also swapped out the 3 pin for 4 pin on a similar set as the OPs as well.

My Q701 and K550 both recabled to have a 4 pin mini XLR connector. They’re both over 10 years old and still going strong. They don’t really leave this room so wear and tear is minimal.