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Thanks for the replies gents. Howard I rephotographed the switch as requested. (below) On this switch, all that was connected to it was what you show as the power lead. When you say the reading light, is that the overhead cabin light you are referring to?? As it has a switch on the passenger side door pillar. Does this switch override that one, so the driver can turn on the light without having to reach across the car??
The unit is tight - that is the rivets on all 3 lead screws are secure and I cannot detect any movement or play in the brass shaft itself. How would one test this unit for diminishing voltage? It sure seems to be a quality built piece; made to last, that's for sure. Also, any idea how I can track down the other 2 missing wires? Like I said, I'm a bit of a puddin'- head when it comes to electrical, but I eventually figure it out! Thanks for your time, (and patience!) much apprec. Chris.

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Looks to me like a broken connection on the back of the switch, where that green corrosion is. There should be a connection between the 2?

Nice work on the tool, Chris. I got one of these for the Jag. You saved yourself $30.
sngbarratt.com/us/#!/English/parts/a970fea8-aaf3-4e05-9ff8-5b64926e440a

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After seeing the second set of photos I am reasonably confident the terminals are identified correctly on the switch. The question now is whether there is corrosion causing the problem or a broken wire. I am leaning toward corrosion as I suspect the wire from the end of the rheostat goes thru the ceramic and wraps around the rivet on the front side where the green blotch is seen. Never having examined a switch up close I can only guess though. You might try cleaning the green area and see if it is corrosion or paint and if there is an end of the rheostat wire that is fastened under the terminal. If it is corrosion and has gotten under the rivet and terminal that could be a significant issue.

Another question would be which is the actual contact for the reading light. There is a small straight line which could just function as the locating tab which keeps the terminal from rotating on the ceramic or it could be that the same wiper that contacts the rheostat wire continues on to touch the metal and make that connection when the reading light is desired. The large round head of the rivet could also be the connection if the large portion of the wiper is also electrically active rather than just there to provide protection and tension for the actual wiper. It could be that both points are used due to the extra current needed for the reading lights. Without seeing the switch can not tell how the center connections to the power in terminal is made as to whether just the wiper is connecting or are both pieces together.

The reading light I am referring to would appear to be one or two lamp assys mounted directly under the edge of dash if this 39 six factory diagram is accurate. No idea if those were standard or optional so they might not be on your car or maybe was removed or connected in another way at some point.

I sort of suspect your switch wiper gets power similar to the later switches where the wiper rotates and a small protrusion on the wiper rides against the solid metal plate connected to the power in terminal. The solid plate on later switches corrodes or oxidizes and the extra resistance as the wiper tries to make contact is enough to lower voltage to the dash lights as if there was another section of resistance wire. That hard to access area is where Ross places the valve grinding compound and I would suggest you also examine that area on yours to see if it appears oxidized.

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Geez Howard - you went to a lot of trouble for that reply - many thanks! I have a lot of points and tips to consider and I will get busy and see what I can figure out. Re those 'reading lights', I do have them on my car and they are on all the time if the lights are on. Hopefully if I can get all this sorted, I can get those switched and the dash lights dimming properly. I'll print the schematic you provided and start identifying which wires are which, and try and isolate the dash and reading light feeds. Thanks again for the detail and completeness of the response. I know I've said it before, but we're sure lucky to have you on this site, Howard. Chris.