Canon T90 issue........not the usual one though !

Have you tried other speeds?

I looked at the actual codes (speed is just the first line): https://en.wikipedia.org/wiki/DX_encoding

100 is: XOXOX
25 is: XXXOX

200 is OOXOX
50 is: OXXOX

The difference in both cases is the second block. So it could be one of the pins is sending the wrong signal.

EDIT: if you put 400 speed film in block two should be black OOXXO. Since the second block is black, if it reads as 400, then its most likely a bad connection for that pin. If its still reads two stops low, then ? maybe exposure compensation?

 

Ink eraser, pencil eraser both work for cleaning contacts as does metal polish.

Likely insulation between the plate and body may have broken down or has been damaged from excessive, repeated pressure in the area.

 

Not necessarily it is a dirty contact pin thing, but it may be an erractic wire connection as with your door switch.
If am right those contact pins are gilded, so you better refrain from abrasive means when cleaning it.

 

An eraser is the least likely of any methods I would personally use to clean the contacts. Isopropyl alcohol and a bud wiped over all of the terminals is a much better and safer idea.

I suspect your camera has been tampered with in some way for the metering selection button to cause a drive advance. There is no custom function on the T90 that can otherwise change functions like those on later pro-level EOS bodies.

The age of the camera, 30+ years, would also be indicative of a full overhaul service. A lot of these cameras were serviced within the first decade for high battery drain and erratic display driver behaviour.

 

An ink eraser is like a fine contact burnishing tool, equivalent to 2000 grit sand paper approximately; a pencil eraser is like 4000 grit sand paper.
Brass contacts corrode and need burnishing at times. A light pressure with either eraser is all that is needed.
Metal polish works best and is least abrasive. Toothpaste (no water) on a cotton swab may work also.

 

I'm inclined to agree with shutterfinger regarding your meter problem. There are several flex circuit boards in that camera. One of them might have become abraded enough in one spot where there is contact being made when there shouldn't be. The fact that you can get the camera to do this when you torque the top plate lends credence to this possibility.

 

The two “hollowed out” pins on your camera are identical to the pins in my T90. While I haven’t used the camera recently, it has always worked properly in the past reading ISO. Though I often bulk load and set the ISO manually. But — I don’t honk those two pins are the source of your problem.