DX derailleur 1990 maximum teeth? I'm trying to upgrade my cassette on a (commuter road bike) bike that uses a Shimano rear DX derailleur from ca 1990. What would the maximum tooth count be for this RD?

The model says rd-m650 but I'm not sure if it is the SS or SGS version? How can one tell? 

It doesn't look like a short cage to my eyes but it's easy to trick ones eyes.

From what I found the it would be either 28T or 32T maximum. Do you think that a 34T would work? I understand that the extremes (smallest-smallest or biggest-biggest) won't work, but I don't use these gears anyways.

Thanks
A quick detour into BING produces the manual from this link:
RD-M650
and partially shown below:


For your derailleur you're correct, 32T is the biggest cog you could have at the rear. It's possible a freewheel with a bigger cog (34T) would work, but you may have to block off that cog by adjusting the limit screws if the derailleur cannot handle it.

I don't recall these derailleurs being available in different cage lengths, they were long cage by default.