Thing about the empire is that it has a good machine milled edge and quite finely etched graduations. This makes it handy as a workshop square because it is robust, light and the full length T edge means it lies flat. Put against an engineering square, you can see light at one end so it's not quite a perfect 90 degrees, but the finest feeler gauge I have won't fit the gap. It's off, but more than good enough for woodwork. I can use it to cut a line with a marking knife, flip, repeat and can't see any error. Steel would be better / more durable but the light alloy is easy to handle.
I have a Stanley adjustable rafter square too. Simply punched from sheet it's super crude and the error at the end of the edge is maybe 0.5mm !
