Theplasticmax: Gill sans is sometimes nearer the mark & I must admit that I had not thought of Helvetica, but from my time in British Drawing offices since 1961, the tracing Departments used 'UNO' stencils & I've not found the font for them, & I'm sure that the MOD (or Min Supply) would have specified the drawing for the instrument company to make them to. Might go back to some pre-WW2 specification for fonts - you never know!
Keith
Keith