I am not sure they are compatible...
See on the picture , with two backs and , on the bottom , some of respective upper spool chamber .
They have a groove but are not strictly similar .
The earlier is on the left ; there is a groove on the spool knob : so , it's possible to close correctly with the back (and in fact the two backs) .
The older back on the right fits the earlier Automat , K4B or MX-EVS ; Automat spool knob has not groove : because of that , it's difficult to close with the new back without damage...
I don't know if it's easy to find now the right older back , but it's possible to cut the two disturbing pieces of an earlier one .