Early versions of Impact used an M4, later ones used M3, as did Control Centres. If you got a couple M4/M3 x 25 or 30, they'll be plenty long enough to trim to length.
All info from the hazy depths of my memory, but i think that's correct.
I'd be inclined to say M3 for that age. (but not 100% sure!)
Early Impacts used to crack the collars, so they downsized the bolt so that there was a smaller hole and you used less torque to tighten the bolt.
