The SO has XT calipers on her bike - they are about 4 years old so I think they have changed. The pads have the hole for the pin at one end, not the middle.
The pin is an allen bolt that you screw in, secured by a mini R clip - or was, until the R clip was lost
All new pads I have seen come with a split pin - is this what I am meant to use as a replacement for the screw in pin? Or where would I find a mini R clip