Sounds like a nice conversation piece if nothing else.....
More than likely tho, someone put it in there after they bought the 10/22. Ruger has only done a coin in the stock on 2 occasins AFAIK (simular to the Winchester '94 commoratives), one in 1976 (as already mentioned) an in a NRA commemorative 10/22 (I am un-sure of the year).
FYI there is now a BSA commemorative 10/22 new this year, no its not the scope brand but The 'boy Scouts of America'. There is no coin in the stock of this model, but the US Mint has produced a silver coin as well, I bet you will see the 2 mated together eventually somewhere....
Cheers!