My appeal for a few measurements of Observation Carriage 150 to check the accuracy of my homemade drawing has been answered - and in spades.
My secret agent went out and took a whole series of measurements along the body and went the extra mile and a half and produced this handy graphic with the figures in imperial and metric.
How's that for service!
The data did show a slight inaccuracy in my drawing, although not what I was expecting.
My concern was to confirm that 150 is longer than a standard Superbarn - which this exercise proved - but I was surprised to find it was around six inches longer that I had drawn it.
I have revised my drawing and now we're good to go.