MK.I and MK.II
Dual Beam Aiming Laser
If You compare both, there is lower amount of material on the heads, around turrets and on the bottom.
Few grams, but its less on rifle nose, so it counts...
Also around output/mode turrets are dimples for turret bearing, so they possitively clicks in place when turned.
Mk.III is even more weight saving and Mk.IV has new QD mount which You know from DBAL-I2