Head harness is very interesting. Mk. 6 Light AG Respirators were fitted with 50s made No. 4 Mk. 2 harnesses, a design seen on Mk. IV GSRs between the two wars. The wartime Light AG Respirators used a thinner strap material and designated the similar looking harness No. L2. The L2 harness was replaced with the No. 4 Mk. 2 harness following the war but in your case, you have the headpad of a No. 4 Mk. 2 with the poorer-quality No. L2 straps, hence the corrections made on the headpad.