Wild was purchased by Leica in 1989.
I own one of the oldest models, an "M5". The M5 was sold new from 1958 to 1989. Like almost all of the Wild scopes, it is not a zoom, but features four discrete magnifications, 6, 12, 25, and 50. This is a very nice progression, with each magnification being double the power of the previous. I find it interesting (and impressive) that the original M5 continued to be made by Wild throughout its history. Used Wild M5 microscopes sell for around $800. I see the M3C selling for $1200.
The M5 scopes have the left and right optical paths with greater separation than other scopes and may offer enhanced stereo effect, but it is nothing that will knock you over.
I also now have an M5A and functionally it is the same as the M5. The power change rotation is a bit more obvious and smooth working.
The M4 model came along shortly after the M5. I have never handled one of these. It was designed as a student scope to be sold at a lower price than the M5 and has a left right sliding magnification changing scheme.
The M3 model came along later yet. The M8 came along at some point and is well regarded.
Pay no attention to any ideas about a sequence of the numbers. They seem to have been chosen entirely at random.
I have never heard whether one model produces superior images compared to the others.
Note that there are Chinese clones of the Wild M3. These are said to be decent and do not seem to be counterfeits (i.e. they are not sold with "Wild" markings). They may be marked Parco or Unitron or who knows.
I have seen M3B, M3C, and M3Z mentioned. The difference seems to be the "magnification changer".
The M3B offers 3 powers - 6.4, 16, 40
The M3C offers 5 powers - 6.4, 10, 16, 25, 40
The M3Z is a 6x zoom - 6.5 to 40
I have an M3C and rather than rotating a mechanism around a vertical axis, there is a knob on the side, which some find more convenient.
The above is with 10x eyepieces and the usual 1.0 objective. Note that a 1.6 objective is also available. The observant reader will note that the 50x power of the original M5 is no longer available and the user will have to be satisfied with 40x.
The following article makes interesting reading. Each model has a number of variants with these being indicated by a letter suffix (such as M5A). Zoom models were produced, designated by the "Z" suffix, such as "M3Z". Apochromatic objectives were available for Wild stereo microscopes, and would be desirable and worth watching for.
Tom's Mineralogy Info / tom@mmto.org