It's not a standard spline. The socket needed to remove the backplate bolts in the photograph is an M10 XZN bit. This is also known as an M10 triple-square bit. Usually you need an extra long bit so you can reach through the access hole in the hub, doesn't seem to apply here. There's a
Facom one here. Clean out the bolt heads with a fine wire brush and then soak with shock and unlock or similar.
The Vauxhall/Opel TIS says to loosen the bolts and pull the backplate forward to make room to insert the new style pins from the backside. As per cam.in.head's post above I repaired a pulled through backplate slot by welding on one of the mating slotted washers from the new style pin kits.
Try to repair the backplates as you'll struggle to find new ones.