That’s an early one you’ve got then. Can anyone else post their serial numbers?? It would be interesting to get a feel for the total number produced each year.

Regarding the windfinding, the unit has two mechanical counters which count up and down in the N_S and E-W directions. Presumably the API gives a pulse output for every unit of distance travelled in the N, S, E or W direction. If one flys a “closed ground circuit” having pressed the reset lever then the final result should be zero readings in the EW and NS directions. The actual reading is due to the effect of the wind on the API. This error can then be calculated on the circular plotter and subtracted off the dead reckoning position provided by the API.