this is all I've found so far
archive.org
All three appear to be identical.
But based on your serial number you can age it up or down from Feb 1968
And here is a snippet view, saying a Proto HD can make 100 lb-ft, just to give you an order of magnitude for that era
a 1974 Construction Methods & Equipment Volume 56, Issues 7-12 showed 95 / 130 (again, not sure its a MKE, but the model number is close
