This is a very small recording altimeter in the style of a barograph. See picture with standard barograph adjacent for comparison. Made by Short and Mason of London, mid 20th C, The clockwork, which completes one revolution every 2 1/2 hours and the recording movement appear to be functioning well. A small and lightweight instrument, it was clearly designed to be supported within the aircraft frame with hanging rings in each corner. Not sure of the scale and have been unable to track down a copy of chart 100. It would, however, be an interesting excercise to work out and discover the scale by testing the instrument in hilly or mountainous locations. 
163mm wide x 92mm deep x  110mm high