If you look at the top of an ejection seat, there are a lot of straps, various, which do different jobs during the sequence. The Type 3 seat also had a wedge-pad which kept the straps and other stuff clear of the face-screen handle.
This pad was conveniently secured by a pin either side of the top of the seat. The pin diameter was of suitable size to fit into the ejection gun sear and could be used to prevent inadvertant ejection. This was the pin I referred to in my post.