I believe that the L vs G designation has to do with European RoHS compliance. Functionality should be the same.
There is no 1200840L1 all by itself. There are two possible options, as follows:
NetVanta 5305 Redundant 120V AC Power Supply 1200840G1#120 Redundant 120VAC Power Supply module for the NetVanta 5305. NetVanta 5305 Redundant 240V AC Power Supply 1200840G1#240 Redundant 240VAC Power Supply module for the NetVanta 5305.
Note that the difference between them is the supported voltage, shown as a part number modifier as either #120 or #240.
Adtran Applications Engineering