12 and 20 gauge are NOT interchangeable with the exception of the older 20 ga guns built on the 12 ga frame which was not done for long and are not frequently encountered. Can't remember when that was at the moment but I think everything since the mid-60's has been on the actual 20 ga frame, which is what I assume you have If you really want an original stock I can't help you there, but if you will consider a new set I may be able to help. Shoot me a PM if so.
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