I recently found myself reading Mason & Dixon and trying to figure out what the rule is for those apostrophes. Pynchon wrote that book in a hybrid of 18th and 2

I recently found myself reading Mason & Dixon and trying to figure out what the rule is for those apostrophes. Pynchon wrote that book in a hybrid of 18th and 20th century English (which is brilliant and possibly also annoying) and boy does he apostrophize. At first it seemed like all the past forms were apostrophiz'd but then I noticed many of them were elongated after all. It appears that the rule is to spell out the "e" in "ed" when it's articulated and use an apostrophe when it's unpronounc'd. I suppose this ought to have been obvious from first principles but I only hit on it by painstaking induction :)