Many thanks for all contributions, suggestions & explanations, both on and
off topic. 
One other informative contribution that I received from Ted Kendall 'off
List' (since he's unable to post at present) follows below. 


We know that Abbey Road was using a falling weight to drive its 78 lathes as
late as 1950, and it is a fair bet that foreign HQs, at least, were
similarly equipped. Now, this system requires some sort of adjustable brake
to trim the speed of the turntable under varying loads, rather in the
fashion of the EMT idler drive machines, and it is my guess that this brake
was simply maladjusted. Odd instances of speed jumps, probably through this
cause, do occur - Peter Dawson's "The Volunteer Organist" has an audible
lurch in the intro - and do you remember the CHARM symposium test-piece four
of us had a go at? If memory serves, that drifted near enough a semitone
over four minutes.

Ted Kendall