It's very crude, pretty much useless for marble games. Maybe it was made for decorative or industrial purposes - like for a lamp pull or claw foot or something.
It's a simple fact . . . not all marbles can be positively identified, and transparent swirls are notoriously difficult since many companies made very similar marbles.
I like the sort of "chevron" patterns on this one - it does look like it got spun on the rollers, whatever it is.
You are really fortunate to have that marble and the letter from your Grandpa. The set is a heirloom for sure. If it were mine, I would do my best to see that it stayed in the family.
Not to say it isn't often cool, but I think horsehair can be just a streak of some contaminant in the glass stream, and that it's usually unintentional. And I'm thinking maybe Heaton for Rick's marble.
It is pretty cool looking . . . looks like it might have gotten hung up on the rollers. I'd guess a transparent WV swirl over a slag but you may never get a positive ID for this one.