Could someone help explain the difference between the Westinghouse OV-20,OV-25, and the OV-50 model cobraheads? I find all of these different models very confusing.

Here is a simple rundown.
The Westinghouse OV-20 is a Clam-shell fixture and most of them look like this. It's an externally ballasted (or remote ballasted) fixture that can take lamps up to 400w (depending on the ballast).

The OV-25 is a Cobrahead fixture and most of them look like this or this or even this(the design of this fixture varied a lot more than the others). Unlike the OV-20, it is internally ballasted (though remote ballasted versions did exist) and can also take lamps up to 400w (depending on the ballast in the fixture).

The OV-50 is the larger version of the OV-25 designed to take 1000w lamps and most of them looked like this (though some had fins and/or glare shields).

Others can go more in-depth about the fixtures as they may know more about them than me, especially since all of the fixtures had many design revisions that others may know more about than me (and there may be something i'm leaving out by accident).

Thank you! I certainly appreciate the help!