I also have a 230 K 6 manual so guess that they are not so rare. At least not in Europe.

 

Mine is also 230 with 6sp.
6sp tranny replaced 5sp after facelift (2001 and newer) and can be found on both 230 and 320.
Also first year SLKs were imported in US (1997) only automatic tranny was available.

Don't forget, that stickshift in general is not so popular in US, when I was buying my car (used) it took me quite a while to find one.

 

I almost bought a 6-speed 230 a few years back. But it had obviously had the snot beaten out of it, so I passed...

 

... and I picked it up!

 

As I recall something between 5 and 10% of Mercedes cars (in the States) have a manual tranny, not really rare, but certainly not common.

Supposedly there are NO SLK 32's with a stick shift, the smallish clutch can't handle it...

 

6spd manual here, too.

 

I found this info on another site that may interest you that shows the production figures of manual and autos for each model
MBSLK.de - The SLK-Community - Produktionszahlen R170