On the B21 the AC compressor dosen't pivot (like it does on later models)

It took the engineers at Volvo several years to come up with a smart way to allow for belt adjustments.

The AC belt adjustment on the B21 uses shims and a split pulley set up. You have to remove the small nuts and bolts that hold on the front part of the pulley to the crankshaft and remove some of the shims that fit between the outer and inner half of the pulley. If you look down there you will see them ( six I think, usually 4 bolts and 2 studs with nuts) surrounding the center crankshaft bolt.

A real pain. A new belt uses four shims, taking out shims allows the two halves to be closer together, making the belt ride higher in the channel and thus tightens it. Usually the unused shims are stored "up front" - slipping them on after the front part of the pulley is in place and before replacing the nuts/bolts. You can do this or just store them. You will need them when you put on a new belt or it will be too tight.

Brilliant Sweedish engineering.