[Different]

Check-Mate Industries made twenty round M14 magazines before the September 1994 federal Assault Weapons ban. Some are marked C.M.I. on the rear side, some are not. The CMI pre-ban twenty round magazines had blank floor plates. The Armalite and Springfield Armory, Inc. logos that are on some CMI magazines are post-ban features.

 

[Different]

1996 production CMI mags have no markings at all.

1996 production CMI twenty round M14 magazines had the LEO/Government Only markings on the rear side. This was done in order to comply with the federal Assault Weapons ban in effect at the time.

 

[Different]

Here's the no muss, no fuss answer. If the following is true:

a) CMI tell tale horizontal line is present on the latch plate
b) there are no logos (Armalite, SAI) on the magazine
c) there are no LEO/Government Only markings on the magazine
d) there are no fake "GI" markings (W, OM, etc.) on the magazine
e) magazine capacity is twenty rounds and it works like it should
f) C.M.I. is or is not present on the rear side
g) the magazine parts otherwise look and feel like USGI quality
h) there is no factory packaging remaining for this magazine

then the magazine in question could have been made before or after the federal AW ban.