Remington made 3 different sizes of 20ga guns.
20ga on a 12ga frame
20ga LW
20ga LT
Receivers and barrels are different between the LW and LT 20ga guns.
The LW uses an 870 style barrel with a short receiver extension, and the LT uses the 12 gauge 1100 style full length receiver port barrel extension.
The LW has a receiver mounted ejector like the 870, and the LT has it on the barrel extension.
The LT also has an assymetrical ejection port.
The barrel extension (part that goes into the receiver) is significantly longer in the LT.
Also the LT has the different shaped ejection port.
Since the LW-20 was made only for a year or so, you'll find the LT-20 much more common
An abbreviated timeline for 20 gauge 1100s:
1963 - the 1100 is introduced
1964 - the 1100 20ga on 12ga frame is introduced
1966 - the first "lightweight" 20, a standard frame with mahogany stocks
1970 - the LW lightweight 20 on the smaller frame is introduced
1977 - the LT lightweight 20 gauge is introduced
Model 1100 serial numbers (on the receiver) started with the number 1001. All but the early guns also have a prefix letter. All Model 1100 Remington shotguns were serial numbered in blocks of numbers. Each serial number has a suffix and the following indicates the meaning:
V = 12ga. standard 2″ frame/receiver
M = 12ga. magnum 3″ frame/receiver
W= 16ga. 2″ frame/receiver
X = 20 ga. 2″ frame/receiver
N = 20 ga. magnum 3″ frame/receiver
K = 20 ga. lightweight 2″ frame/receiver (LW)
K = 20 ga. lightweight 2″ frame/receiver (LT)
U = 20 ga. lightweight magnum 3″ frame/receiver
J = 28ga. 2″ frame/receiver
H = .410 3″ frame/receiver
https://www.remingtonsociety.org/collecting-remington-model-1100-shotguns/
