Hi Jeff - How are you?
Looks like I have the same Vivitar 3x teleconverter. There is also a 2x in the stash I inherited from my dad. Never used either of them, but I know they are also used in close-up/macro work. (I use primarily a macro bellows) You've got my curiosity sparked though so I might play with it over the holidays to see what it does. He had it epoxied from a screw mount to a smaller screw mount (step-down?) god only knows what he was doing with it. That's the way he flew with things! Let me know how it works out for you. CHEERS!
Not withstanding the optical loss there is the problem of holding 1200mm steady. That is 24X a 50mm lens so each tiny vibration is magnifed 24X and it is a bear to try to hold steady even on a heavy duty tripod. What in the world is anyone shooting with such a rig. Perhaps small birds at moderate distances but I think applications for that long a focal length would be very limited.
In the 70's aftermarket 2X and 3X were plentiful and even then, cheap. Mostly they were bought, used one or two times, and then banished to the bottom of the gaget bag. That is why so many old ones look like they haven't been used.
I never saw a 3x tele-converter. I only learned about their existance from as test in a US photo-magazine from ca. 1970 where the 3x ended worst.
But if one needs that effect that moment on the lens one got, the technical quality of the image is of lesser interest.
Ok, so I googled and came across this article using one of the old Vivitar 3x TC's.
I have to say, I am rather impressed with the results.
Most of the reviews I read were negative for the 3x, but not the 2x for the reasons stated by posters above.
I realize this person tested with a d***. But, I like to tinker about a bit with my gadgets too so, needless to say, I will play with my old beat-up 3x TC over the holidays. :)