In 1970 the first American Z's arrived, with the 2.4L engine. I had serial #2189, built in May of that year. At that time, they were all 4-speeds. Soon we discovered through the competition preppers that there was an excellent 5-speed available on the Japan Fairladys based on a ZF design; for one thing, it had Porsche-style synchros. It could be retrofitted into the early Z's with some effort, involving custom propeller shafts and rear end mounts. Never personally knew any Z owner who did it though.
Did the 5-speed become standard on the Z some time before the 260 or 280?
Just checked, because I couldn't remember... mine has 160MPH on the original speedo. I've only seen 140, with new tires, on a road closed for a rocket launch, somewhere near White Sands, many years ago. ;)
It also has the big throat carbs, gets about 8-12 mpg, but gets it fast. My original owner's manual (I do still have that) doesn't show a 5 speed manual transmission for the Model S30.
/johnny