Steven -
I've been throwing 13 pounds for years and have had to do a lot of investigating.
I was told by Dynothane that their blocks are the same down to 12 pounds.
I just purchased a 13 lb. Vendetta Black Solid and really like it. It's very user friendly, easy to control, predictable and can be used on most lane conditions. I suggest you get your mother a 13 lb ball as I have found it to be a very comfortable weight and seems to hit better than my 12's used to.
I don't know what your mother's style is but any one of the Dynothane's would be a good choice. Their 13's average between 13.0 - 13.4 oz.
In addition, a lot of women in my ladies league have the 13 lb. Original red
ebonite Predator. They are all doing great with that ball. All their averages
have gone up. Most of these women don't have any hand and are throwing a lot of 200 games with them.
I hope this helps.
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LadyWannabe