It is not part of an Alpha (SS) water pump replacement. I was just mentioning it as being one of the few 'water flow' parts different than the regular Alpha.Again, this piece is in the upper drive where the tube bolts too. I'll be in my garage working on some sleds in a bit, if my Alpha SS is reachable I'll take a pic as it is in half right now for, a drum roll, impeller/impellar housing, and etc replacement.

​​​​​​​Yup. That is what the manual states too.
Sorry to throw you off, my way of explaining things can suck thru typing.

It should help. Most I know that installed one gained 6mph +.

Using same prop, mph killing mufflers/same tune, the 20ft boat with approx 350hp-370hp I work on went from 65-72mph., just changing the drive. We'll work on more speed when time permits.

Okay, here is the upper half with tube. Under where is the 4 bolts, is a metal piece that is 1/2 closed for the Alpha SS. As Ryan mentioned, this is supposedly to cut down water pressure as the Alpha SS has low water pickups which can create a bunch of water psi at higher speed. The regular alphas have this gasket/plate under this tube totally open to water flow.

Sorry for the bad pics. I'm not so good at taking them, but atleast I took them. Usually I forget. LOL.

One last thing, with Alpha Gen1's and Alpha SS, when you split the drive into halves, the oring at the top of the vertical shaft will get stuck in the oil seal in the top half. Very easy to get with a pic. The oil seal is in the pic's I provided above. Just slowly stick your finger in their, the oring will rotate enough that you can get with a pic or small pliers. If the oring is still on vertical shaft, I have never seen this. LOL.

Gen II's got rid of this oring and groove for it as the groove for it is where the vertical shaft breaks, if it does.

Here is a good step by step on replacing the Apha Gen 1 waterpump. 'A' on Water Pump Removal step 2 is the oring I was talking about.
MerCruiser Outdrive Water Pump Replacement Instructions - MerCstuff.Com