Gravely-power: I have a couple of Darling Brown&Sharpe items that I inherited from my Grand father. One is a draftsman's triangular scale made of duraluminum. The other is an architect's scale made of the same material. I haven't seen any others so I don't know how rare they are. They both have their original boxes.
The Draftsman's scale:
The architect's scale:
Very nice pieces. I have both in my collection. Your architect's triangle is very nice. I have never seen one with the original sleeve. I follow these items pretty close. If you want a value just post, which I prefer, Or pm me.