The single piston Girling calipers were introduced on the 1991 turbo 700/900 cars, and used through the production of the 900 series. Being a 1990 model year, your car would have originally had the dual piston calipers. Your right side strut was probably damaged and replaced with a later unit. The left side will have a 11.25" diameter rotor (known as the Big Rotor), and the right side will have a 11.00" diameter caliper (known as the Jumbo rotor). The Jumbo rotor is substantially thicker than the Big rotor. The surface area of the single piston is nearly identical to the surface area of the two pistons. The dual piston Girling Big brakes take the 508 pad, and the single piston Girling Jumbo calipers take the 1198 pad

You need to match the brakes on both sides of the front end. I hope the car has ABS and a single brake line to the dual piston caliper. If not, you'll need to do some more work. You have a safety issue.

My first Volvo had mismatched front brakes, and it was scary in emergency braking, going all over the road.

There is a nice section in the 700/900 FAQ on brakes. See the part on converting to Jumbo brakes. I wrote a lot of the information, and supplied the pictures.

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john