Cast iron square with center on sale head

This is a very unusual cast iron square with center head. Research was difficult. No manufacturer's name. It appears that this piece was made up by riveting two iron squares together. A careful look at the sides of the place where the squares come together for riveting, shows an almost undetectable line where the two join, plus the rivets which are obvious on sale. I hope pics show as much of this as possible. I do not think the set screw that tightens the rule in place is original. It is a puzzler to me and I have been doing this for 64 years. No ruler with it (a substitute was used for pic.) but as you know new rulers are available.
$65 includes shipping.