As you can't find your calipers, I'd suggest getting a strip of paper, wrapping it around the stem that fits, marking where it overlaps, lay it out straight, measure the distance and divide by Pi. It seems very unlikely an MTB of that vintage would have anything but a 1 1/8" steerer which would mean a quill stem diameter of 1" (typically called a 1 1/8" stem because they go by steerer dimensions).