Hopefully KevG will put up a photo! But basically, the L4 bipod sleeve has a steel shroud that extends rearwards over the gas cylinder vents. Simply to shield the brighter flash that comes from the 7.62mm blast venting from the gas cyl.

Off the subject - sort of for a minute....... This was a bit troublesome on the Navy L4A3 and 5 guns because the ferocious blast was directed rearwards onto the blast plate which was a separate part on the ex Mk2/A3 and 5 guns. The blast plate was meant to be fixed solidly to the gun body (through the medium a tapered cross-pin) but the blast could cause it to work loose. The inspection standard was relaxed by the NOD so that slight looseness was acceptable providing that......... And off it went on and on and on.

Now where is KevG with the photo.......?