Cast iron popular square with center 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. 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. popular