My Minolta Rokkor 100-200 f/5.6 also has a maximum aperture of f/5.6 as the name suggests. I suppose this is the lens you saw, as for that price the 400 APO would really be a steal. Besides - ahem - where did you see it?
The 100-200 is a peculiar beast. The two zooming lens groups move in parallel, keeping a constant distance between them. The lens has an unusually low number of lenses for a zoom of the time. Very crisp and contrasted, but also showing a jolly good amount of distortion, this is something you should not use for architectural work for sure (OK you probably wouldn't use a zoom lens in any case). The small maximum aperture makes it somehow outdated by today standards. When extended, it really looks like a bazooka (that's because of its peculiar optical scheme, as said) so don't use it in a war zone (unless your wife hates you, and you love her, that is).
As far as I remember, the MD series of the SR mount was launched with the first "multimode" camera, the Minolta XD-7 in Europe (whatever else was called in other markets). The lens needs to be set to its minimum aperture (marked green) when the camera is used in shutter speed priority mode.
When the X-700 was launched the MD series was modified receiving a lock on the minimum aperture. That way, somebody wanting to use the X-700 in Program mode only could lock the lens on its minimum aperture.
As a curiosity it is interesting to say that the behaviour of the Program mode of the Minolta X-700 (and probably of the shutter priority mode of the XD-7) can be modified by setting an aperture which is smaller than the minimum one. If, for instance, you set a normal lens to f/5.6, the result will be that the Program mode will select f/5.6 and 1/1000 for EV14 @ 100 ISO, but will overexpose at EV15 not being able to close the lens more than that.
At EV10 (ISO 100), and with the lens at f/16, the resulting exposure is 1/125 @ f/2.8;
At EV10 (ISO 100), and with the lens at f/5.6, the resulting exposure is around 1/250 @ f/2.0 if the lens is capable of f/2.0, but it reverts to 1/125@f/2.8 is the lens is not capable of f/2.0.
This is explained by the graph at page 30 of the Italian manual.
With my 100-200 f/5.6 the Program mode would keep the full aperture until EV14, and then begin closing the aperture with the usual slope (7 diaphragm stops and 4 aperture stop for each 11 EV intervals).
Fabrizio