I take it you are referring to the Diana/Original 45- if so there are 2 plastic bushes set into either side of the stock- and these hold a steel pin captive between them that passes right through stock and the trigger mech as well, thereby holding the action in the stock.
Get a similar diameter drill bit or even an oval or round nail and drift this pin out which will then release the stock from the action.
After a number of strip downs, these bushes can become a little worn and in extreme cases can result in too slack a hold on the steel cross pin and its possible loss whilst gun is in use. I think some gun part suppliers still have some of these in stock.
Good luck with the strip down- I have one to do one of mine in the medium term to fit a ptfe head.
Dave