sfmans: "So on early ECs the relevant notes (D# / Eb, A# / Bb, and G# / Ab) were originally tuned differently? Any idea which temperament was used, and when the decision was made to switch to equal temperament?

RE: Like I said to "tinasqueezer" above there seems to be some misunderstanding here.It was not so "originally" but the english keyboard seemingly turned out being very well suited for being used with mean tone temperament favouring the different D#/Eb and G#/Ab.
(NOT the A#/Bb). Call it a "spin-off" maybe in modern terms but not as I said intended from the start.There was NO "decision to switch" and strangely enough mean tone tuning was obsolete in other music enviroment even earlier except that (British) pianos also were late in that transition ( mid 1860s or so) to equal temperament

"I'd obviously heard of 'Old Pitch' before (although mainly relating in concertina terms to anglos rather than ECs) but hadn't picked up that extended to temperament as well ..."

RE: Old pitch ( = "Old Philharmonic pitch" a=452,5)has been in use with all concertinas and longer in relation to Salvation Army or Concertina Band instruments likely since military brass band kept it longer and maybe since it had been used for organs and harmoniums as well.