Kinda ashamed. The bolt was an early Group Industries conversion bolt by he two fasteners on the sear plate. Downside is that cut on the RHS that will likely cause trouble.
I am not an expert, but it looks to me like the bolt would be an unregistered conversion bolt. The listing has been updated to state the bolt is not included in the sale, so my opinion of the bolt may be correct.
I messaged him last night to ask if the bolt was registered. It most definitely is an illegal bolt. He has since told me he destroyed it and is only selling the other parts