I am 90% sure I need the 135mm QR version as I have QR on the hardtail. Though I am curious what is the difference between say the QR version and the 10mm through axle.

choice of axles

I had assumed there was one axel size and it was the end caps that made the axle compatible with different setups?

Though I am curious what is the difference between say the QR version and the 10mm through axle

Er, one is bored to take a QR skewer and the other a 10m through axle? Is this a trick question?

You can use QR, 10mm bolt-in or 10mm bolt-through but the QR one is the standard. Bolt-in sounds like it is, and bolt-through you’d need a 10mm rear spindle.

(to explain why- your rear dropouts are 10mm, but you know how usually the wheel actually sits into the dropouts with 2 sticky out bits? So that even if the QR is out, the wheel still sits in the dropouts and doesn’t fall through? Those are 10mm. The bolt-in and bolt-through doesn’t have that, and uses that extra thickness for a fatter spindle.

I had assumed there was one axel size and it was the end caps that made the axle compatible with different setups?

thats the deal for front hubs. rear hubs need different axles.

Although the new Pro2 EVO is said to use just end caps to effect the change in axles.

What is the difference between 12mm end caps, 10mm end caps and normal QR end caps?

My 142mm x 12mm (X-12) rear conversion kit only has one end cap on the non drive side, the drive side is the axle.