The designations date back to the days of Black Powder and I believe there were such rounds as the .45-70, .45-90, .45-100, .45-110 and .45-120. To my understanding, the numbers after the "-" did represent the powder charge, but there are those here far better versed in this than me.
I'm attaching a freebie from SWAT October 2001: Clint Smith's GREAT Article on the History of the Big Fifties.
Rich
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