In the studies in the History of the English Language II there is an interesting article on the merger of the vowels /ɪ/ and /ɛ/ before nasals as in the word pairs, “pen/pin” and “Jim/gem” in American southern speech. Vivian Brown seems to argue for a late (1880) development of this merger, but these two authors, Michael Montgomery and Connie Elbe, argue that it could date earlier, and most interesting, that it might have an origin in England! Or, equally interesting to me, that the merger may have begun in African American speech in the South.
Regarding the merger in England (both the land and the word?) they cite English Dialect Grammar (1905, Wright) which notes that in some 19th century dialects of the British Isle, there is a short i before nasal consonants in such words as “hen, men, pen, went...” Is this still the case? Then along came a “correctness” movement at the end of the 18th century and into the 19th century that suggested (as in the 1823 edition) that “engine” pronounced as “ingine” is only among the “most vulgar speakers ... which cannot too carefully be avoided.”
So, was this separated by prescriptivists and then merged again in the American South? I don’t have the above cited book at hand, just searching through it at Amazon.
There’s a longer article on “high front” and “weak” vowel mergers at Wikipedia, but it depends only on Brown.