[Coda]

Originally Posted by Kaderin Triste

Well....unless Montana is divided into vastly different linguistic regions, it's not a regional thing. Same as me he was born in MT and has lived here all his life. And does not replace any other long E sounds with long A.
It's literally just windowseal/windowsill.

Update: he has informed me that it's actually a medical thing where his frenulum I think it's called, is farther forward than most people, so it's easier to do vowel sounds where the tongue doesn't have so much vertical movement. Apparently he also says "pellow" instead of "pillow".

That's curious. Maybe this is just a matter of my accent, but the word "sill" (and others, of course) doesn't involve lifting my tongue off the floor of my mouth at all. The vowel shape is all done at the back, so the frenulum doesn't come into it. (Trying it out for a bit, it turns out that the tip of my tongue stays behind my lower teeth for every vowel I can think of. That seems curious; the linguist in me wonders if this is common or if it's part of my particular accent.)

But limiting the range of motion DOES impact the production of the L, which could be relevant here. I can imagine that someone who speaks in an accent that doesn't use a dark L in these words might have a problem shifting articulation between the vowel and the L smoothly, which might change the shape of the vowel in the process.

(To define "dark L": There's a curious consonant made with the tip of the tongue down and the back of the tongue almost as far back in the throat as it can go. It's sometimes described as "make an L the same way you make a W." Polish has an L that's so dark that in modern use it's indistinguishable from an actual W, but in English use the dark L is still recognizable as an L, in the accents that use it. Imagine the stereotypical New Jersey "I'm walkin' here!" The L in "walking" is a dark L. By contrast, the L made with the tip of the tongue against the roof of the mouth is called "light L".)