Does anyone know what type of socket to use with these?

A 10 mm 12-point socket is too large and a 9 mm is too small.

I suspect that it is some kind of imperial size.

Sounds like 3/8 unf will do the trick. Probably is a unf size as they are American based...

Thanks,

Will try that

It was 3/8"

The bolts are in place