Now this gets confusing, I will tell you what I know (which I think is right).
I think the 20/20 on the barrel is 20 Ga, 20 inches long.
There is a 870 20 Ga. Std
There is a 870 20 Ga. LW
There is a 870 20 Ga. LT
The Lt's are no longer made, but do require a special barrel of their own. The LW and Std barrels are not interchangeable. My understanding from asking Remington this question is that all new 20 Ga are LW's even though they do not call them "870 Lightweights" anymore. So if it is a new gun (do not know what year) the current 20 ga barrels should fit it. But the LW's will not fit the old STD. Confused yet.
If you have the instuction book it should tell you if it is a lightweight. Bottom line, call or E-mail Remington with the model number or seriel number because I have asked differenct barrel vendors at gun shows and always seem to get a variation of the answer Remington gave.
Doubt if this helped, but hope so.