I referenced that forum's webpage when I first got involved with repairing the Unity spotlights, and it is very informative, but it does have its shortcomings in a few details. If you search the word "Unity" along with my name "Paul Steinberg" in these forums, you will find additional information available in the Technical Forum.
To be very cautious about, the head post assembly, shaft assembly, and handle assembly are not universal, even though they all look the same. Through the years, Unity made small changes to these parts, that render them noninterchangeable. I learned about this recently after looking for the correct length shaft for my 1962, only to find that even though I now had what I thought were all the correct parts, the gears didn't match up. Even shaft diameters changed through the years and are not interchangeable with the handles and head post housing assemblies. The lamp head assembly appears to be the only part that is universal and interchangeable through the years.