Hello there,
Check omps2 from Brescia region .I am not sure about their shipments to America during pandemic but certainly they still stock many 121 M1 parts .
I just got the same shotgun and came across this Berreta and Benelli parts and accessory supplier based in Italy .I am still looking to find a proper disassembly diagram or manual for my M1 121 .Good luck...
https://www.omps2.it