Strange accessory belt sizes

Hey all,
My March is getting to the point where it needs some new accessory belts, as the ones on there when I bought the car are getting a bit too stretched and squealy on startup. My car is a 2000 with aircon and power steering. My aircon/alternator belt is a 3PK635. This is easy because it's printed on the belt. The cooling/power steering belt doesn't seem to have a size printed on it, but I took a very approximate measurement with a piece of string and found it to be ~960mm externally, with 6 ribs, which would make it a 6PK960.

Nowhere I look online has mention of this belt size configuration, and I'm worried about buying the wrong cooling/power steering belt size with such an approximate measurement. Everywhere I look mentions different belt sizes.

Does anyone have any idea what size belt I should be after, or a reliable resource I can reference?

Don't think it's relevant, but it's a 5 speed automatic, not a CVT, which I gather is pretty rare.