This outstanding find is exceptionally rare and complete.
Boresights were used to align the gun so that it would be accurate in use.
The early models of the ME-109's had guns carried inside the wings, from the Messerschmitt Bf 109-F series onward, they were outfitted with a 20 mm gun firing through the propeller shaft.
The change was disliked by leading fighter pilots such as Adolf Galland and Walter Oesau, but others such as Werner Mölders considered the single nose-mounted gun to compensate well for the loss of the two wing guns.
In place of internal wing armament, additional firepower was provided through a pair of 20 mm MG 151/20 cannons installed in conformal gun pods under the wings.
This boresight comes in a box that is marked 'Ziellinienprufer f. Gesch. 2cm Kal.' Had this been for a cannon, the designation Kan for Kanone would have been indicated.
The large box measures abuot 24 inches by 7 inches by 4 inches tall
Inside the box you have the actual bore sight with mounting bracket. The optics are usable and the reticle is still present. (Not easy to get a picture of but my picture is included)
In addition to that are two separate glasses that are perfectly clear. One is clear glass, one is slightly tinted.