You can see that the channel for the cocking rods are milled open in this exploded view.
I think the difference is age related, not gauge related. The receiver I provided pictures of is one of the 1st 1000 produced. As I stated to Kirk in a PM. They made so few 20ga guns that I doubt they forged a separate receiver for them.

AH-10 = Firing Pin, Rear (2 Req'd)
AH-11 = Firing Pin Seat (2 Req'd)
AH-13 = Hammer, Left
AH-14 = Hammer, Right
AH-15 = Locking Bolt
AH-16 = Locking Plate Screw, Long
AH-17 = Locking Plate Screw, Short
AH-18 = Lock Plate, Left (Marked A.J. Aubrey)
AH-19 = Lock Plate, Right (Marked A.J. Aubrey)
AH-2 = Bridle, Left
AH-20 = Mainspring (2 Req'd)
AH-21 = Mainspring Lock Screw (2 Req'd)
AH-22 = Safety Thumb Slide
AH-23 = Safety Rod
AH-24 = Safety Rod Screw Post
AH-25 = Safety Spring
AH-26 = Safety Spring Screw
AH-27 = Sear, Left
AH-28 = Sear, Right
AH-29 = Tang Screw, Long
AH-3 = Bridle, Right
AH-30 = Tang Screw, Short
AH-31 = Top Lever, 20 Ga.
AH-31 = Top Lever, 12 Ga.
AH-31A = Top Lever Arm
AH-32 = Top Lever Pin
AH-34 = Top Lever Spring
AH-35 = Trigger, Left
AH-36 = Trigger, Right
AH-37 = Trigger Pin
AH-38 = Trigger Guard
AH-39 = Trigger Plate
AH-4 = Bridle Screw (4 Req'd)
AH-40 = Trigger Spring
AH-41 = Forend Iron, 20 Ga.
AH-41 = Forend Iron, 12 Ga.
AH-42 = Forend Wood, 12 Ga.
AH-42 = Forend Wood, 20 Ga.
AH-43 = Forend Latch Assembly
AH-6 = Cocking Rod (2 Req'd)
AH-7 = Extractor, 20 Ga.
AH-7 = Extractor, 12 Ga.
AH-9 = Firing Pin, Front (2 Req'd)
CO-12 = Firing Pin, Front & Rear
NI = Barrel, 12 Ga., 32", (Pinch Spring Type Forend Catch) Marked Meriden
NI = Lock Plate Assembly, Left, NI (Marked A.J. Aubrey)
NI = Buttplate Screw (2 Req'd)
NI = Lock Plate Assembly, Right, NI (Marked A.J. Aubrey)
NI = Barrel, 12 Ga., 32", (Pinch Spring Type Forend Catch) Marked Meriden
NI = Extractor Screw, NI
NI = Action Spring Forend, NI
Pete