Finished latest project

That is not a US Vickers Tripod. It is a Turkish Vickers Tripod.

I have an identical one in my garage right now.

I don't know about the plate on the back leg, but...

a Colt Vickers tripod has the tripod head locking clamp in the front of the tripod base. A British tripod has the head locking clamp on the right side. The length of the legs of the Turkish tripod are longer than either the US or British ones. It is also lighter and somewhat more delicate in construction too. It is my favorite tripod to use when shooting my Vickers.