Greetings, marbles can be all kinds of fun! It looks like you have a mix of vintage marbles - I would guess from the 1930s-60s, maybe later. I see Akro, Vitro, Peltier, Alley, etc., and maybe a few newer ones from Asia.
It looks like some type of impurities - it's not so uncommon to find stuff like this in a marble -unless it's actually mica in an old hand-made. It is much less common when you see it in the form of a patch, or a ribbon, like that last Pelt you showed.
I think the categories NLR, MCS & Rainbo are imperfect, and I really have no idea how people distinguish one from another. The blue and yellow on white one you're showing is not a MCS, and I don't think Rainbos have six ribbons, I also don't think folks will call it an NLR so . . . I am hoping someone who knows the differences will give us both a good explanation!
I just read this explanation on the Peltier Info Site, "Peltier Rainbos are a later development growing out of the NLR line. Many of the same colors are used but the main difference is in the ribbon NLR ribbons are laid across the surface of the base where Rainbo ribbons extend into the interior of the base glass."
So maybe it is an NLR, and the number of ribbons has little to do with anything.
The one color swirls like #3 are often hard to ID since several companies made ones that are nearly identical. I think you should check for AV in that blue one, since I think I see a little green leaking out of the blue, and it does look like it could be an Alley.
I couldn't find my original comment so I will reiterate here. What I hoped to say is that it's harder to find rainbos with clear transparent glass bases and colored ribbons than it is to find those with colored transparent glass bases and white ribbons. But it's fun to collect the white-ribboned ones in colored glass because there are some colors that are harder to find than others - like aqua, and chartreuse.
I like the white-ribbon rainbos with a colored transparent base. They look nice in jar, and there's a good variety of colors too - some much harder to find than others.
Welcom to the Marble Connection. I'd say it's a Vitro Conqueror type, but a number of companies made marbles that can look similar - a good look at the seam you don't show straight on, might help with a more positive ID.
If there is a lot of "feathering" on the underside of those white ribbons (like someone dragged a fine fork through the underside of the white striping glass) on the inside of the marble, it might be CAC, but 3/4" would be a much less common size for CAC than Alley, and CAC didn't make anywhere near the numbers Alley made either, hence my hesitation to go there. Either way, the shape suggests it's probably dug.