Try your local gunsmith. The Riverside is a Stevens shotgun. If you can find out which model it is, you can probably find the wood at Numrich Gunparts
I have a 1913 model 16 ga riverside arms shotgun . all it says , is the date made and company so on mine there is no ga. indication .
riverside arms co. split single shot gun
Stevens/Savage used the Riverside name from about 1920 to 1945.
Can't give you an exact date, but Stevens used the Riverside name from about 1920-1945 according to my notes.
The Riverside model number will be identical to the Stevens number.
By looking in the chamber.
1958 was the first year that the Automatic 5 shotgun was made for 3" shells. This was for 12 ga. guns only. 20ga guns were available with a 3" chamber beginning in 1967.
Riverside Arms Chicopee Falls Massus. (569HF
$75
try looking at savage or Stevens shotgun diagrams. I found a firing pin for a 16 gauge riverside it was an old model 94 Stevens part
Riverside was manufactured by Stevens, so if you can identify the equivalent Stevens model, you should be able to find parts. Check with a local gunsmith.