I noticed that the depth does seem similar to the post's drum.
Perhaps it was an approximation of a metric depth? I do have many more Tromsa and Trixon shells around than Sonor and even though each of those companies officially went Imperial in 1959( same as Sonor) and 1963 respectively, the depths of the shells on the early Trixon 14's stayed metric for a while and the Tromsa 4 x 14's of which I have a number of examples are quite variable.They can be anywhere from 3 1/2 to 4 . It seems that they went from 90mm in the early years to 100mm later but all the while called the drums 4 x 14.