Once, after teaching a session on auxiliary verbs to an ESL class, I remarked to a friend on the construction "might could", which I take to be a southern idiom for what another might express as "maybe could" or "might be able to". The friend, who grew up in Virginia, came back with "used to would". I had never heard this, but another ESL teacher, a native of Tennessee, said that she had. I take to "used to would" to be a slight modification of "used to"; but whether it strengthens or weakens it, I don't know.
This past week I was interested to find to "used to could" in The Pioneers. I suppose that "could then" or "once could" would convey the same meaning. Natty Bumppo, who uses the expression, is represented as a native of New York, therefore distinctly a northerner. I think that Mark Twain was mostly correct about Cooper as a novelist, but I imagine that Cooper knew how people spoke in upstate New York as 1800 approached.