The photos below are copied from Ned Schwing's book on the 21.
General Omar Bradley's custom grade 20 ga. 21 with a Hession style forearm. Same length(more or less) as a field or skeet forearm, but much smaller across the radius; Hessions could be made as thin as manufacturing would allow and gripped the barrels tightly on the sides approximately even or slightly below the center line of the barrels. Approximately 9 to 9 1/2" in length...I have a skeet grade 12 with a 9" skeet forearm.