[et] was the normal past tense of 'eat'; it was spelt either 'eat' or 'ate'. I don't know when the pronunciation [eit] ("ayt") came in for 'ate', but I'd guess very recently, later nineteenth century, and from the middle class in London, since [et] hasn't been fully displaced in either upper or lower class speech.

I don't know what the age/class distribution is in US or Aus/NZ: I _think_ I've heard [et] in Australia by older people but can't remember. Did they ever say [et] in old US films?

An edition of Spenser from around 1900 or a tad earlier actually glossed 'ate': it said simply 'did eat'. So the very spelling might be recent (in non-dialectal).