[Matti Salonen]

The EKM shown by Andreas is the version used by everybody before March/April 1940. It had an inscription of the unit in clear text, which of course was good for enemy when interrogating POWs.
It was changed in 1940 to an encrypted version, which was used by flying personnel in fighting units. Flying personnel in homeland and ground personnel continued to use the old version.
In a new system a MOB number was used to hide the actual unit. Normally every staffel had its own number(s) (as 67009 for 4./JG 53). In the beginning the maximum was 20 persons for every MOB number, but that limit was quite soon omitted and at the end there might have been hundreds of individuals for a single MOB number.
To make things more complicated also letter(s) were used with the MOB number and often it is difficult to know whether letter A or B belongs to the EKM or if it is a blood group.
The MOB number had 5 or 6 digits. About 3000 different MOB numbers were used in total. Fortunately most of the EKM catalogs survived and are now at WASt.

Matti