|If I were you, I'd ring Moyer Marine and ask them; they know A4's like no one else on earth. You'll get the right part, as quickly as a delivery service can send it.
BTW, if you have a sight glass on your filter, replace it. Even a small fire (something you could put out with a hand extinguisher) could rupture the bowl and turn a nasty problem into an inferno. The US Coast Guard doesn't permit them, with good reason. Don't know how our CG feels, but I'd go with the US rules here and get a metal sediment bowl.
C&C 27 Mk IV