Are you sure it's rubber ball? If you look in the holes do you see a 3/4" shell, then core? It's been over 35 years so the memory is fuzzy, but I think the Brunswick Mantas were LT 51 seconds. Brunswick came out with the LT 48 rubber and LT 48 plastic which was green and black. They stopped making the LT 48 green/black plastic and came out with the LT 51 blue plastic ball.
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