As the A10 is a bike regularly fitted with a chair this is what I guess they are for.
Yes, the A10 was a favourite for sidecar use and has a hole in the frame for just that purpose.
I fitted (with the help of a friend who was a motorcycle salesman at Kings of Oxford, a well known Motorcycle dealer at the time) a double adult Canterbury to my A10 in the mid 60s (the GF in my photo).
We fitted in in the cobbled back lane at the back of my parents house. When done, and after giving me advice on how to ride/drive it, my friend said take it up the lane and back to see what you think but you'll need more throttle than usual. Having only ever ridden solo I jumped on, kicked it over, put it in gear, let the clutch out and opened the throttle, shooting forward with a tank slapper that nearly put me in the wall both sides of the lane. My friend had forgotten to tighten the steering damper down.
Rog.