21 mm is .83 inches. At that size, yes, the likely options are Jabo or Mexico for modern, Alley for vintage.
With the glossiness, modern does seem likely. But I'm not sure which direction to go from there. Possibly leaning Jabo but really not sure.
I hear your assurance that it is clay. I still can't see that though. The high shine doesn't go with clay in my mind.
It looks mineral.
Normal clays are worth very little even in mint condition.
Typically we will want to see views of seams. You're showing one in this image. Getting clear looks at both seams of a two-seam marble can be helpful in making ID's.
Lighter?
Could be normal clay, but it's larger than usual for basic "commie" ... the most common clay marbles.
The shininess is also unusual for an older clay marble. But that could mean little to no play wear, I suppose. If it is common clay, then don't wash it. I've had the colors come off when I washed clays.
I agree with your reservations -- not a Bennington.
Is your sense that it's a lighter or heavier marble?
From the pictures I'm leaning toward a newer, more dense clay than a Bennington would have.
That's a neat bit of history.
Disappointing developments but you can display it all together in a Riker case. Still a nice thing to have in your collection.
Marbles are likely Vacor. It would be cool to have that header. Would be interesting to eventually learn if there is a date on the header, but would be nice to have it no matter the year.
I'm missing something. I'm referring to the sharp shooter bag header. That looks "real" and interesting to me.
Your avatar IS beautiful if that's what you thought I meant.
William, is yours metal?
There are some nice precision calipers which don't have metal. I haven't researched or invested. I actually do use metal, but I worry about damaging the marble.