Wow! just received this beautiful old Tapley meter as a birthday present. Beautiful piece of kit. superbly built and intriguing dials.
There is a little light at the rear.
I have no instructions how to use the meter, but later meters were set up in the vehicle by aligning the rh sector with the "level" mark and the lh to "0". Then accellerate the vehicle up to at least 20mph and apply the brakes. Then the resulting lbs per ton would indicate brake efficiency. I seems Tapley meters are still made! This is their website with even more information. This specific instrument is not listed anymore, but I have written to them to ask for more information.