Because this particular 6 pdr tube was used for experiments after the war. A series of 6 pdrs were rebored and rerifled to different calibres in 2 or 3 mm steps from 59mm to 73 mm. Ammo was then made and tested to see if they could get a better compromise between the speed of the 6 pdr (57mm) and the heavy shell weight but slow speed of the 75 mm. This particular one is more like 67mm than 57 mm although the outside remains completey accurate 6 pdr Mk V dimensions. The gun was bored for pressure testing so can never again be fired, even if the special ammunition could be duplicated. One other thing, no it is not missing a muzzle brake or counterweight they never had one. Some had a thin sleeve to protect the threads but many did not have anything.

Thanks to John Pearson who owns the tank