Some Master Marble observations:
MM are two-seam marbles. Seam characteristics: generally one seam will have a "V" or convex projection while the other seam will have a concave "U" indentation.
Sunbursts are generally the most collectible. These are the "onionskin lookalikes" that can resemble Akro Sparklers but usually (not always!) have fewer colors. Some sources say that Sunbursts always have transparent base glass (not always true, there are opaque Sunbursts); to my knowledge everyone agrees that the base is filled with filaments of varying colors that run, roughly, pole-to-pole. Sometimes there is a larger "filament" that may look more like a patch. The transparent base glass may be clear, red, green, yellow, or blue (these are the colors I've seen.) Filament colors include brown and grey along with yellow, green, red, blue, white, and orange. True (unblended) pink is rare. My experience indicates that red-based Sunbursts (the deep MM red) are probably the most rare.
Master Marble Sunbursts can often be distinguished from the later Master Glass Sunbursts by their more brillant and distinct filament coloring. I don't know for sure but I'm guessing that the light blue-, green-, and yellow-based Sunbursts may be MG. As John mentioned above, Master Glass was a step down in quality. Some of the MG brushed patches are attractive but you might have to look through a ton of them to find the really nice ones.
Master (MM) also made very striking two-color patch marbles, Comets and Meteors, which are very similar to some Akro patches. MM colors are a bit more subdued that the Akro colors which can seem almost garish. These MM patches may also be differentiated from their Akro cousins by the considerable (and highly attractive) color blending whewre the patches meet. These marbles come in at least seven different color combinations.
Regarding marble size: I have three diameters under 5/8" and three over 11/16" in my collection. The largest I have is 1 inch, the smallest 17/32". But I have very few in these sizes.
Please feel free to send any unwanted or neglected bags, drawers, or tubs of Master Marbles on over to me. Just let me know and I'll give you the address. I promise to cherish them.
Sword44