Simple......... it is the fume extractor from the L8 and L37 et al tank derivatives of the GPMG. I seem to think that there was a small tube part in the kit that extended backwards to the gas block area and the venturi effect draughted the exhaust gasses forwards and out of the tank. The ex tankies here (jonnyC?) will tell you that the gas from the co-ax machine guns are more overpowering than the main armament. Mind you, they fire a lot more rounds than the main armt!!!!!!!
Tankie and Skippy.......?