There are three basic types of AK selectors. There is one for milled receivers ("AK-47"), another for the AKM, and one for the AK-74. The milled receiver variant is made from thicker steel than the other two, and is "slab-sided". By contrast, both the AKM and AK-74 selectors have a distinctive longitudinal "stiffening rib". The AKM variant has the same sharp corner at its top/rear as the milled AK variant, while the AK-74 selector has a rounded corner. Chinese Type 56 rifles use either the AK type or (surprise) the AK-74 type.
In addition to the AK/AKM/AK-74 pattern differences, the dust/top cover on the rifle generally dictates which selector one should use. For example, a Romanian SAR-1, although an "AKM", uses an AK-74 selector, due to its "flared" dust/top cover.