Took a closer look at the top shock mounts. All is not as it should be.
This is the r/h side:
The threaded part of the mount can be 'wiggled' slightly and is held in place by a pin driven through - the end is visible halfway along the shank.
Drove the pin out and we have this:
The mount consists of a hollow section, like a hollowed-out bolt, welded from the inner side of the frame. Pushed into it a crudely-made mild steel threaded insert secured by a bent pin. Hmmmm.
Perhaps not a show stopper.
The shock mount needs to be secure - any weakness is dangerous and any give will, uhm,
compromise the handling.
But if the hollow welded part of the mount is secure, then the function of the threaded insert (aka bodge) is to hold the securing nut and prevent the shock leaving the action sideways. In normal use, there is no obvious source of lateral force to push the shock sideways. So the securing nut is not doing a lot of work.
If my reasoning is correct, then it will be safe to re-assemble with the new shock. But opinions from the good and wise people on this forum are
very welcome . . .