What 50mm lens are you talking about that has enough room on it that allows for a tripod mount? Or even needs it for that matter? Many weigh in at 3/4 lb, the heaviest at 2 lbs. Even the 2 pounders do not have tripod collars.

The Tamron 70-300 weighs 27 oz and can be used with the camera attached on a good ball head. Tamron would have added a collar if it was necessary (such as they do for the 40 oz 70-200). I have a similarly sized Sony 70-300 and it does not have a collar. Check out Canon and Nikon, they do not have a collar either.

I bought a used Tamron 70-300.It seems pristine except it lacks a tripod mount. So I checked to see if there is such a thing as a tripod ring mount that is adjustable so it will fit more than one size of tele lens. I couldn't find one so I sadly assume that I must seek a tripod ring mount that is specific to this particular Tamron lens. Bummers!

==>Before I start searching the stores for ring mounts specific to each of my tele lenses, am I correct in assuming that adjustable mounts do not exist?
..........................................

Addendum: I just returned from checking BH, Adorama, & several other stores. None of them had tripod ring mounts for my specific lenses. Good grief, what does one do in such a case... get someone to make mounts for me or what? Uh I suppose I could use the elastic cords used to secure surfboards, bikes, & such to my bumper. Or is there a better way to cobble a substitute for a ring mount?

P.S. lesson learned -- never again buy a long lens package that doesn't include a tripod mount.

I bought a used Tamron 70-300.It seems pristine except it lacks a tripod mount. So I checked to see if there is such a thing as a tripod ring mount that is adjustable so it will fit more than one size of tele lens. I couldn't find one so I sadly assume that I must seek a tripod ring mount that is specific to this particular Tamron lens. Bummers!

==>Before I start searching the stores for ring mounts specific to each of my tele lenses, am I correct in assuming that adjustable mounts do not exist?
..........................................

Addendum: I just returned from checking BH, Adorama, & several other stores. None of them had tripod ring mounts for my specific lenses. Good grief, what does one do in such a case... get someone to make mounts for me or what? Uh I suppose I could use the elastic cords used to secure surfboards, bikes, & such to my bumper. Or is there a better way to cobble a substitute for a ring mount?

P.S. lesson learned -- never again buy a long lens package that doesn't include a tripod mount.

Every photographer should have one:

https://www.harborfreight.com/6-in-digital-caliper-with-sae-and-metric-fractional-readings-62569.html

I paid about $8.00 for mine on sale and it is remarkably accurate and reliable. For a machinist, perhaps not (although +/- 0.001 compared to some at 10x the price), but I use it daily and keep my expensive one in a drawer. Many sellers (or reviewers) of aftermarket lens collars on Amazon will give an exact inside diameter of the ring. If not, ask!

This is why some ads say "will fit such and such lens only". What they don't want you to know is that 70.5mm = 70.5mm (or whatever) regardless of the name on the factory door.