Thanks for your reply!I'm not certain, but I believe there were 2 different deck styles used on the Original over its production. The early style, which is the one I have for my O, is a 2-spindle timed mower deck. The blades have to be held at a specific position with a timing belt so that they don't crash into each other. There is no PTO mounted on the engine, rather just a pulley. The PTO is a sliding plate with a pulley stack on it, mounted directly to the mower deck. When engaged, the pulley stack slides out and puts tension on a regular drive belt, driving one of the spindles. power is then transmitted to the other spindle VIA the timing belt. This deck has its own specific mounting setup for the Original, and probably wouldn't fit onto a 70/100 without some modifications.
However, I think, in the later Originals, they had the option to come with a mechanical engine-mounted PTO and a Cast-end 38" 3-spindle deck. This deck is NOT timed, there is no need to align blades with a timing belt and it is therefore much simpler. It is the exact same style deck used on the 70/100 tractors, and I believe it would work with a 70/100 without much modification (if any).
Hopefully someone will better verify this, I don't remember exactly where I read this from.
Edit: here's a photo of the 3-spindle deck on an O. Perhaps this helps to verify:
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