The action is a "square-shroud" target action that came before the Coopermatch Series - this action is what Malcolm Cooper used primarily for 300m target shooting. Easy to tell by the "AI S/S + serial no." stamping. Some later ones had "AI SS + serial no." without the slash, but the even earlier ones had "CWC/SS + serial no", meaning Caig+Walls+Cooper. Coopermatches are clearly stamped as such on the left and are visibly a bit different.
Lones Wigger and Harald Stenvaag also had these actions. The square-shroud action is essentially the same as a L96 or PM without the magazine well and safety, so the original bolt would have looked like a PM or L96 bolt.
Looks like the bolt has been replaced by an AE bolt. Whilst a 6 lug Coopermatch bolt with the round shroud could probably be made to fit with a 3 lug boltface installed, they're not fluted, so I'm inclined to think that the bolt in this rifle is from an AE which had 3 lugs, fluted body (not sure), and the same round shroud at the back. The boltknob is also smaller. The original one would have been teardrop shaped and Coopermatch bolts have a larger boltknob.
The trigger looks to be a later type and that "US" stamping isn't from factory.
Very nice rifle with an interesting replacement bolt! Compatible with modern day AI chassis systems if you can get your hands on a debonded one.