There has been a bit of discussion about the differences between the L1A1 to A4 bayonets recently. Here is the official documentation taken from EMER, SA&MG V150, issue 1, May 1971. Page 3, paragraph 13.
L1A1 and L1A2. Introduced at the same time, these models have a crosspiece held firmly by two rivets against a shoulder on the haft. The pommell is also rivetted in position. The difference between the two models, in addition to amall dimensional differences is in the method of grinding the cuting edge. On the L1A1, the cutting edge is achieved by grinding equal angled flats on each side of the blade.

On the L1A2, the thickness is reduced in two stages. The larger flats having an included angle of 40 degrees and the flats ground to the edge having an included angle of 55 to 60 degrees. Amendment 1: This arrangement is to allow the manufacture of the bayonet by outside cutlers having regard to the best and most efficient use of their existing machinery.

The bayonet catch on both models protrudes from the pommell approx 1/4". The protruding catch was liable to be operated acidentally, causing disengagement of the bayonet

L1A3 and L1A4 To overcome the defect in the L1A1 and L1A2 bayonet, shorted catches were designed and the bayonets modified to accept the new catches. After modification, the bayonets were redesignated L1A3 and L1A4 respectively. Further defects concerning loosness of the cross pieces and pommels were corrected by introducing brazing as an ADDITIONAL or ALTERNATIVE to rivetting. To further ease manufacture, slight dimensional changes are permitted and although the marks are not advanced, a differences will be apparent. The grooves on some blades are shorter in length and with the grips removed, some shafts will be seen to be a different shape.

Amendment 2 dated (unclear on paperwork). GRIPS, bayonet, pairs are marked with the identifying data relating to either the Bayonet L1A3 or L1A4 and are fully interchangeable. As such, these are not indicative of a sub mark of bayonet. The bayonet will be referred to in (should read 'as'!) its generic description of BAYONET, Rifle, L1A1
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