"Rattling" is something loose. "Sloshing" is altogether different.
Post some pics, but if the previous owner did all that, sounds like he set it up as a target gun, so some form of recoil reducer in the stock seems more and more plausible.
Sorry its taken a while guys but i found there source and you're right. Looks to be a recoil reducer. The actual reducer is stuck fast in there, but what was making the noise was the wood dowling seen on the table which is free to slide back and forth about an inch or so.
problem solved. Next problem...how to add about an inch in LOP to the gun (cheaply)!
British used ebony, Bakelite, hard rubber, or hard tight grain walnut for stock splices.
No effort to match the stock wood, an obvious extension between stock and India rubber recoil pad.
If you are good with wood you could add a grain matched addition to the existing stock or a contrasting addition. What is the length of pull with the existing pad? What is the distance from the inside of your elbow to the first joint of your trigger finger? Ideally those should match up closely.