The early looms were issued, or rather delivered without plugs, earphones or masks/microphones.
When assembling the looms for use, I can imagine they used whatever was present: black and brown plugs.
Also, when a loom was damaged, it would be logical that a newer loom was fitted to replace the old one, retaing the rest of the hardware.
A cracked brown plug (easy to do with bakelite) on an old loom could've been replaced with a black plug.
So I would accept the brown plug is the earlier type, notwithstanding the fact it was fitted to later type looms (or vice versa).