[Landpimp]

yes you can, actually that rear mount is correct for a G3(and the optics and mounts were made for G3's not 91's), if you use the HK claw mount/Hensoldt(as some come) you normally have to remove the rubber bumper to use on a 91/3/4/etc

other mounts may(I dunno) made to 91 not G3 specs