Dr M said:
...
all I know is that my tractor is called a....1500. Is that considered a YM155. It's a 2 wheel drive, 2 cyl, 17hp.
...
Well, since no others have spoken, I'll pass on what I've read, heard, and surmised from previous discussions in the forum.
It is my understanding that the 4 digit models, i.e. YM1500, are models that were built in Japan for the Japanese market and are present here (USA) as a result of the "grey market" in used tractors. They were not intended for sale in the US and are not equipped with all provisions necessary to meet the consumer protection/safety standards for new tractors sold in this country; manuals, placards, etc. are in Kanjii (Japanese) script, controls frequently operate reversed to US custom, no PTO guards, no ROPS, etc. Furthermore, Yanmar's position relative to those tractors imported into the US by 3rd parties (the grey market) is that they, Yanmar, will not support those tractors either with parts or repair from any Yanmar authorized source.
The 3 digit models, i.e. YM155, are models built for US (and other countries?) markets and are built in accordance with the applicable customs and regulations; English markings, manuals, etc. They are not the same as the 4 digit models although I am sure there is a lot of parts commonality between similar models. These models, as I understand it, are supported by Yanmar.
I suspect there is, somewhere, a listing that pairs grey market (4 digit) models with their closest US (3 digit) cousins, but I'm not aware of where one might lay hands on a copy. I don't believe there is any significant correlation between the first two digits of the model numbers. In other words, I don't believe you can count on a YM15XX being similar to a YM15X.
As I said, this is just what I've picked up from discussions in this forum. If I'm wrong on any of it, I hope some better informed person will step up and offer corrections.