This is a L1A4 bayonet, on account of the long fuller and plain (as opposed to 'waisted') crosspiece. L1A3 bayonets with the plain crosspiece had the short blade fuller. The "C" marking is common on L1A4 pommels, however, the "AM86" marking much less so. Some speculate this to indicate manufacture in 1986. However, I haven't seen anything to substantiate that (it is possible, just can't say that it is fact). In an earlier time, "AM" signified Air Ministry. However, that went by the wayside in the mid-1960s and the L1A4 did not enter production until 1973.
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The bayonet cannot be abolished for the reason, if for no other, that it is the sole and exclusive embodiment of that willpower which, alone, both in war and everyday life, attains its object.—General M. I. Dragomirov