It is a SWA gland where the cable attaches to the metal clad CU. The solar PV installer has cut away the outer sheath from the SWA to fit a BS591 clamp onto the armour instead of just retro fitting an "earthing nut" or a banjo washer.
These Official Forum Sponsors May Provide Discounts to Regular Forum Members - If you would like to sponsor us then CLICK HERE and post a thread with who you are, and we'll send you some stats etc