The Mercruiser Dry joint exhaust came out in 2002-2003 range and newer motors. If your engine is before 2002, you do not have factory dry joint exhaust.
Dry Joint was only used on 4.3L GM V6 engines and GM 5.0L, 5.7L, and 6.2L small block V8 engines. If you have a big block 7.4L 454 cid engine, you do not have Dry Joint Exhaust! With that being said, let’s determine if you do have dry joint exhaust.
Dry Joint Exhaust used only two water ports to transfer the water from the manifold to the riser.
Non Dry Joint exhaust used 4 water ports surrounding the exhaust port to transfer water to the riser.
Since you cannot see the ports with the riser installed take a look at the riser itself:
With Dry Joint, notice the cast elbow on the front and rear of the riser:
With Non Dry Joint, notice no elbow casting in front or rear of the riser:
With this information you should be able to determine what you have. Dry Joint and Non Dry Joint components are not interchangeable!