I seem to recall that at one point the Teletype Corporation offered an hours meter for at least the ASR33. Still does not answer though why on earth an IMSAI would have one installed considering it would of cost pennies to operate and a few dollars to repair if it was a warranty thing.
We have some rack mount Variacs at work, with hour counters on them. This thread has reminded me to wrangle them up and get them ready for sale.
That seems like a lot of hours on that IMSAI but we don't know what was already on the counter when it was retrofitted, nor how long it was used before the counter was added, so I guess it's more of a curiosity than a source of information.