Yep, that's the check valve. Super simple design, but it works. W
I wanted to take a minute to comment on oil usage in both the Alpha's and Bravo drives as it seems to be a topic that surfaces on the forum almost weekly. My contentions based on 35 years in the Lubes business and over 40 years in boating are as follows.
First of all, I feel that any actual consumption of oil by a drive system indicates a leak. If the drive shaft bellows accumulates gear oil, the input shaft seal is leaking and needs to be changed along with a careful inspection of the top bevel gear bearing set. If the owner is very aggressive about greasing the gimbal bearing, then it would be normal to see a small amount of carrier oil separation from the grease accumulate in the bellows, but it would not be the same color as the gear oil.
Secondly, the remaining three drive seals are all below the water line during operation and a leak would be evident by an oil sheen in the water both during operation and at rest. For obvious reasons, these also need to be fixed in a hurry due to pollution issues and also their ability to draw water into the drive housing during cool down. Other than that, there are a few gaskets and an o-ring which I have never seen fail unless they were installed improperly.
This leads us to the question of "why does my drive use oil, and where does it go?" Based on my experience I suggest the following.
Gear oil is a complex hydrocarbon matrix comprised of large chain molecules which are subject to a phenomena known as shear. Think of a gallon bucket full of tennis balls. You can only get so many balls in the bucket due to the amount of air that is surrounding each ball. Under heavy loads like water sports as you suggested, some of these molecules break down into smaller pieces due to shear. Now the bucked seems to be only 7/8 full as the tennis balls are now smaller in size, but there are more in number and if we were to weigh the bucket we would find that it weighs the same as before. We did not loose any balls, that just take up less space then they did before. The same thing happens with gear oil. As the molecules shear, they take up less space and it appears that there has been an external loss of oil. Actually it is still there, just degraded in quality somewhat. After 50 to 100 hours of this abuse, it begins to loose its effectiveness and is in need of a change out.
Gear oils have a very high density, and a fairy low gravity. Drop a golf ball in a bucket of gear oil and it take a while to reach the bottom. Drop the same ball in Gasoline and it sinks quickly. Now weigh each bucket and you will find that the gasoline is much heavier then the gear oil. Gasoline has a higher gravity, but lower density. That’s because its individual molecules are very small with little space for air between them. The same volume of gear oil has a lesser mass as the complex chain molecules are large by comparison and have a great deal of air between them. Beat those big molecules into little ones in a high speed, non-hypoid gear case such as an out drive, and pretty soon the same amount of oil takes up less space, and needs a top off. Nothing actually leaked out, the balls just got smaller.
Lastly I'd like to suggest a variation in the method in which you and others change your drive oil. I keep reading about the air that gets trapped during refill and the subsequent need for topping off, and also about drive oil reservoir level alarms after servicing, and I can tell you I have a quick and easy solution. DON'T REMOVE THE TOP VENT PLUG, or install it tight prior to refill. When draining the drive, remove the cap from the remote reservoir, and remove the bottom drain plug. It's important to drain the reservoir anyway as sediment accumulates in the bottom of the bottle due to thermal capillary circulation during normal operation. Get this old oil out with the rest of the drive oil as it is just as old and just as worn out. Attach your filler to the bottom drain as normal, and pump up the drive until oil appears in the remote reservoir. Look Mom, no air. The oil tube that runs up to the plastic bottle on the Gen II's exits the drive higher then the top vent plug, thus displacing the mystery air bubble. Using this methodology, I have never had to top off a drive after servicing. If you beat up an Alpha, expect to add a little oil due to shear, but if you are doing that often and there is no external leakage, the drive oil is shot and you need to think about changing it more often anyway. I use a pressure tank to fill my Bravo III-X as it take over three quarts. I change it every 50 hours, and have never had to top a drive off because I don't give the oil a chance to begin to shear and I don't start out with an air bubble in the first place. Keep the top plug in and watch out for shear loss and I think your drive will be around for another decade or so. I hate to hear our forum members talking about top off on drives without level alarms. If you have to add a half a quart to a Bravo, or even less to an Alpha, then the top bevel gear set and bearings in the upper gear case are already taking a big hit. Hope this helps.