OK...yea, that is the book I was thinking of, probably I had the 1st edition or so...too many holes, I wouldn't buy it again, sorry...maybe he has gotten better, someone speak up and put me in my place!
As to your power transformer date code. "926" is the manufacturer, "129" is the coding for the date that it was built...the power transformer. 29th week of something ending in "1." Well, we know it cannot be 1951, and certainly isn't 1971, so it has to be 1961.
I used to have a listing of the manufacturer codes, but cannot put my finger on that now...
So with only the info here, we know the amp cannot be any older than July(?) 1961 as it has a component with that date in it. The potentiometers give you another clue/date and the can cap and maybe even the eletrolytics may give another clue/date if they have not been changed out.