Another of the unusual items I have discovered during the cataloging of my mothers stuff. Not sure what it is or whats its for. Makers name I believe is Hill Bros or it could be Mill Bros. Its difficult to see as the first letter and the beginning of the address seems to have been worn away in use. So I guess its held at this point whilst being used. There are no other markings either on the brass body nor the steel movable piece. Its very simply made with the brass part measuring 8" long X 1/4 " thick. The steel part is 9" long x !/8" thick with no graduations, movable and lockable via the thumb screw. The ends of the steel piece are shaped but not sharpened as would be if used as a scribe. Interesting find! Has anyone any idea what it is and what it is used for?
I believe that it is a brass masonry square. After looking up what you said was marked on the antique it's seems this is true as there are links to other antiques being sold under the same name. In addition if you wish to look up antique masonry tools(squares) you will find devices that look similar the one you have.
Oh also everything I found about the marking on the side and the meaning of it point to the device being made at "234 Wandsworth RD, London" under the manufacturer name "Mill Bros".
This is a stonemason's 'sinking square' - used to determine depths of mouldings, carvings, etc. on a piece of worked stone. Mills Bros were manufacturers of mason's tools, being situated in Wandwsorth Road, South London, an area which was the centre of the capital's stone working industry until the 1970s.