Well, good news and bad news. First the good. I learned why some have said that there is a difference in the sizes of the holes in the tach/speedo compared to the oil/volt/temp/fuel gauges. The (4) smaller guages have a smaller "tube" formed on the back of the gauge, versus the wide open hole with the green diffuser in the tach/speedo. Also in comparing the holes, I noticed that the tach had perfectly clear green tinted diffusers and the speedo diffusers were either burned or are just filthy as Tinster has stated. These are extra ebay gauges, not my own, which I believe to be in much better shape.
Now for the bad news. The bulbs have a slightly bigger globe or lens which will not fit in the (4) smaller guages, so it looks like it's back to the drawing board and another call to the lighting company.
I'm still going to try to swap out bulbs in my TR6 over the weekend to compare brightness, or if I can get sockets, light up these spare guages wired to a 12V power supply.
The Mr. Wizard experiment continues after a short commercial break..........sigh............