Looks like they took off one of the gate paddles, it was a Wey & Arun original that I rescued from Malham Lock and restored to working order.
I also remember helping make those gates, they came if I remember correctly from the GU at Watford, we took them apart cut off the rot and reassembled as we needed narrower gates and not so deep, but that was easier said than done the Oak was so well seasoned it was black so cutting and drilling it took a huge effort.
I shall attempt to find my photo archive