Conrad-Johnson Discolored (rusty?) Allen screws on side of MF-2550.

Those appear to be socket head cap screws that are often made from black oxide alloy steel.   The black oxide process is used to add,

"mild corrosion resistance, for appearance, and to minimize light reflection. To achieve maximal corrosion resistance the black oxide must be impregnated with oil or wax."

Moisture in the air (humidity) can contribute to corrosion.  Humidity present in lower levels of conventional homes (i.e., in basements) can condense on cooler surfaces.  Increasing air flow, increasing temperature, and operating a dehumidifier all help reduce problems related to humidity and condensation.  However, even mild condensation over years could have contributed to the oxidation you see, it is not uncommon on those types of screws.

If you are concerned, open the top and look for any obvious evidence of corrosion inside (be safe - unplug the amp first and do not touch any large capacitors or other areas which could result in shock).  If you see significant evidence of corrosion inside, take a picture and send it to CJ for discussion. However, if everything looks fine then I wouldn't worry about it except to make sure you are operating the amplifier in a warm, dry place.   If you wish, you can replace the screws or clean and oil them as discussed by @russ69.

Thanks. I'll check it out.

I'd be inclined to just get a bunch of rustproof screws and replace them but I want to first find out if they have to be a specific kind of metal and what they are fastened to. I'd hate to mess up the job of re-fastening new screws to components inside the amp since it's working fine now.

I also noticed something strange. At the exact same position on both right and left sides (the fifteenth cooling fin), there is an empty screw hole with threads showing. Since it's at the same position on both sides, it doesn't look like an accident. Maybe the case has the threads but they're not used for this amp design.

I hope I can get an answer from the company without having to bring it there for service.

OP a little dab will do you. Put a little light oil like 3 in one or lighter WD40 on a soft fiber or even brass bristle tooth brush size brush. Gently brush the heads of the cap bolts. Just get them wet.. Leave it alone. Let it set for 20 min or so.. Two or three swipes two or three times.. They will look like new AND be ready to remove it you want to do so..

It's not the space shuttle..

LOL BUT that is how you would work on it too :-)

Wipe off any excess with Isopropyl. Touch up the heads with a permanent marker, touch up paint or nail polish..

Regards

If you use oil, no wd40 no regular oil.

   Mobil 1 synthetic. Just half a drop anywhere needed will be great.
1 quart willlast a lifetime.