In Germany you won't find a 143 too often as well.
There is also 133 super extra and the 143 mx.
If you consider the "extras" a separate model or not, may depend on the national market.
In Germany the designation "extra" was used to differentiate the professional models from the ordinary range. So "extra"-saws included 110, 115, 115i, 116si, 120si, 133 super, 143 and 166. Later PS-6000i, PS-6800i and PS-9000 were members of the "extra" models.
In other markets, it might have been different. I once read a bulletin for north american Dolmar dealers where a certain product enhancement was introduced to the 133 super and the enhanced saws were to be designated 133 super extra.
A little bit the same for the 143 mx: These were AFAIK all 143 after the transfers were altered in 1988 but the "mx" wasn't stressed equally in all countries.
If you would be a little picky, then you could also differentiate between the regular solid state ignition and the optional point type ignition, that was available for some of the big SACHS-DOLMARS.
So if shelf space and money weren't a problem, eben the few big ladies would offer enough variations to keep a collector busy for quite some time...
I for one am happy with what I have so far. I really love these powerful saws!