[ei] is used (systematically, in RBP*) in two cases that I am aware of; there may be more. These are my two:
1 in current speech, where the speaker knows that a noun is coming, but hasn't decided what it is yet, or whether it will take an adjective, what that adjective will be, and so on. It's a way of signalling to the listener 'Don't interrupt, but I need a bit of thinking time'.
2 to emphasize that the thing being spoken about isn't unique: "Yes, I had a knife, but you can't prove it was the murder weapon".
I remember a joke that used this device. It was set in an audition for impersonators:
"Next... Name please."
"Right, that's A. Hitler"
"No, the Hitler"
* there may in fact be dialects that prefer one pronunciation or the other,
but this is essentially not to do with dialect
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