Those were used up to about '99 or so, then Volvo put out a TSB that they should be removed, because they could melt from an overheat and block the exhaust, and they felt that they were not effective in keeping water from coming up the exhaust. They were a hold over from OMC where Volvo used OMC's transom mount design during the joint venture and used many OMC design parts. I have the same ones on my engine, and have not had problems with them, except I did have to replace them due to an overheat once. Personally I did not agree with Volvo on this point, because I never had water come up the exhaust on this boat. I feel that some kind of flapper is needed on sterndrives, if not this design (which is very similar to Mercruisers who still use this type of flapper) them maybe a larger flapper like that used on inboards, in place of the exhaust bellows. I have seen a few Volvos set up this way, an inboard style flapper installed on the exhaust opening of the gimble housing.