1942 RAF 20mm Cannon Sight Testing Instrument

Another first for me is this 1942 dated RAF Sighting Instrument for the 20mm cannon.

Made by Avimo who had a long association with government optics, this instrument is complete with its original storage case measuring 640mm x 210mm.

The principle of operation was to slide the long arm into the barrel of the gun and then the armourer could look down into the eyepiece, a prism in the long arm allowing him to see the exact aiming point of the cannon via a simple cross hair on a target. I assume the aircraft would have been raised to a position replicating level flight.

Overall condition is very good with both the Sighting Instrument and the storage case although two of the divisions within the case have dislodged and need to be glued into place.