To be honest I can't see the slot in the barrel that you talk of. But the bottom barrel looks like a variant of the Mk2 barrel that was produced for cheapness, eliminating the positioning guide that you can see in the top photo. In truth, this positioning guide/channel and the locating peg on the gas blast plate served no real purpose because you couldn't position the barrel wrongly in any case. If you tried, it wouldn't go into the gun! And when the barrel was in the gun and locked down with the barrel nut, it was as tight as a ducks bottom in any case.
But we reverted to it in the Mk3 and L4 series. Just another part of the strict licensing agreement that we adhered to, virtually to the letter