I wonder..... There was a couple of years overlap back when Yanmar USA closed up their dealerships and began building compact 4x4s for John Deere. That was the era of the JD650 through 950 compact models. Those were also offered with an add-on power steering kit that was said to be similar to Yanmars, but better. It was even more costly. In fact, I thought about adding the kit designed for the YD650 to my own small Yanmar, and even went so far as to make some measurements. Those notes are long sense lost, but I do remember that I thought the JD design was applicable to other tractors and superior to the Yanmar kit.
Could this one you are showing pics of on your YM276 be one of those JD power steering kits?
rScotty