Cases were always Nickel plated brass as far as I’m aware, unless some very early ones were steel.
Mk3, mk4 numbers were stamped on the cover, and not normally on the case.
As already mentioned, covers and dials were often swapped (especially Army bases & MoD) so give no reliable indication of the lock itself.
It was developed and progressed from the original Milner Merlin lock, an example of which is top of Brian’s Museum list if you know anyone who has one :-)
the oldest I have isnt anything special at only 1958, but it does illustrate many of the later developments when compared to say a 1990s or early 2000s example.
