Tractors go by "model run" rather than by "model year" like cars do. A model run for a tractor might be made all at once during the first year and then boxed up and warehoused to be sold over the next howevermany years the model run goes for. If they run short, there will be a supplemental model run and that's often when accumulated small changes are included. The best place to sort the various model runs out is a late model parts manual.
For awhile I collected Yanmar literature and manuals - partly because we had two of their tractor. But also also simply because I was intrigued by the quality of the manuals - which might have been among the best ever done by a tractor company. For their workshop manuals they would start with paper back provisional workshop manuals when the models first came out, and maybe a year later would publish the final workshop manual - typically that one is the best. It's recognizable by being in a heavy ring binder with red plastic covers. The final workshop manuals are great, featuring lot of hand-drawn exploded diagrams, color foldouts, flow diagrams, a theory section, and with multicolor printing on glossy stock in a special red plastic folder.
BTW, congrats on finding your Yanmar. I've been active on Yanmar forums for about 20 years or more now.
Someday I still plan to end up with a YM186/187d.
rScotty
