I will drop the 'gauge man', Taffy Txxxxx a line and see what he comes up with.
I think that the L1 rifle part of the designation is just the generic term used by us Armourers as a means of identifying the rifle generally. Like we'd say '...... have you finished the L2's yet.....'.
Talking of L1's and L2's.......... I wonder if anyone has noticed the HELIX ruler. Makers of L2 plastic pistol grips and large batches of the plastic part of the plastic L1A1 handguards