I had 12ga 31L and 16ga 31L and looked at 20ga 31L. The 12ga was heaviest and 20ga lightest. Interestingly none were 'L' marked but frames were alloy. The frames were gauge specific. Interestingly the barrels in late 40s 12ga were fully interchangeable (I had 3 barrels from same production period as the gun). The barrel shanks threaded into steel insert in the alloy receiver.