I can't find a horizontal shaft Greenfield in the manuals section either. The mini-tractor is the only one that has a pulley and clutch system that looks like yours, and as you said, it is vertical shaft so there will be some substantial differences.
Contacting Greenfield seems like the best next step, especially if you can quote the serial number of your mower. I find a lot to like in the Greenfield design, at least with regard to the chassis and propulsion system. it would be nice to see yours - an unusual model - sorted out and documented for our archives.
