Correct, i think cats came in for diesels in about 1994. 1992 for petrols.

Because of this the early 306 TD didn't have a cat so if you can get an exhaust from a scrap yard for one of these it'll save alot of trouble and you can still swap back to your original with cat for MOT time. - If yours is an early one and you do fit the exhaust with no cat it may just pass mot as the tester might think "oh, it's an early one with no cat so thats a pass even though emissions are out.."