I have 32" barreled field guns and duck guns, both are plain (no rib) barrels.
They were made with ribs or you could get one installed. One of the 32's is part of a 2-barrel set, the second barrel is 30".
You're right about the 30" barrel being much more common then the 32". It seems that the plain barreled, full choke 30" 12 gauge Model 12 was the standard issue of the day.