It's hard to say, they were never standardized. Check your motherboard's manual for the pinout if you can find it, or figure out where the + and - pins are with a multimeter then do some trial and error to find the data and clock pins on your motherboard.

My question is more about the socket - if it's the same socket and just more wires... I'll take the 8-way one and clip the ones I don't need.
Also, this item looks like it has 5 individual "sockets" inside, which doesn't look like a typical PS/2 socket.
I think I am better off getting something else...

From my experience, even if the holes on the PS2 bracket's connector physically match the PS2 header on the motherboard, the pinout might not be the same.

To be safe, you need to look for the PS2 header pinout in your motherboard's manual. Then, use a multimeter with continuity testing to determine the pinout of your PS2 bracket. And finally, rewire the bracket to correctly match the motherboard header (e.g. VCC to VCC, clock to clock etc). Here's a helpful video.

Yeah this I know... (thanks for spending time to take care that I cover that, though!)
Just wondering about the "holes".
Isn't this connector more suitable than the one I previously linked: https://www.ebay.co.uk/itm/304016597873 - this one's holes look more like I am used to for PS/2 and they are 6, not 5... I think I'll go for these (better value too)

DraxDomax wrote on 2021-06-21, 02:41:

Yeah this I know... (thanks for spending time to take care that I cover that, though!)
Just wondering about the "holes".
Isn't this connector more suitable than the one I previously linked: https://www.ebay.co.uk/itm/304016597873 - this one's holes look more like I am used to for PS/2 and they are 6, not 5... I think I'll go for these (better value too)

PS/2 always has 6 holes, even if only 4 are used.

DraxDomax wrote on 2021-06-20, 23:51:

My question is more about the socket - if it's the same socket and just more wires... I'll take the 8-way one and clip the ones I don't need.

That won't work. The Mini-DIN plugs (like the one on your PS/2 mouse) are specifically keyed with plastic pins to prevent insertion into "wrong" sockets, i.e. sockets with more pin holes than the plug is designed to mate with. If you want to plug a mouse into an 8-pin or 9-pin Mini-DIN plug, you would need to remove the plastic key pin from the mouse plug. 6-pin Mini-DIN sockets should still be easily obtainable, so I don't see a compelling need to improvise there.

so, when they say, in the first link "8-way" - that's the number of pins (a different socket shape), not just more wires coming out the back?
(I did wonder how a 5 pin socket can break out to 8 wires!)

DraxDomax wrote on 2021-06-21, 10:32:

so, when they say, in the first link "8-way" - that's the number of pins (a different socket shape), not just more wires coming out the back?
(I did wonder how a 5 pin socket can break out to 8 wires!)

correct.