C50-N18L-A3 lawn mower battery

I have a C50-N18L-A3 lawn mower battery as shown here:
(Amazon.com product link shortened)
As you can see, it has 6 yellow "corks", plugs, one for each of the six cells, with which to add water when needed, I would think.
But it also has, and the picture shows a trace of, another plug in the channel closer to the middle of the battery.
What is this one for? To add water to all six cells at once? To add water to some other chamber? What is the other chamber and how do I know if it is low? Or is there one?
I haven't had this second-hand battery long and it took me a long time to even see the water level, which is substantially low. I have a gallon of distilled water, but I'd like to understand the seventh cork!
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On 04/30/2016 05:22 PM, Micky wrote:

As you probably know, I was in the industrial battery business for 38 years. One of the big problems I saw was people over-filling their batteries.
You need to charge the battery first and water it immediately when done. Do not fill it up into the neck region.
Just enough to cover the plates and then a bit more.
If you fill it first, it will almost for sure boil over when you charge.
The exception is if it's "bone dry" , you will then have to add water first...but only enough to just barely cover the plates.
As to the 7th hole, I've never seen that, please post a photo of the top of the battery
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I've been pretty good about that, but I've only had car batteries before this.

The battery has marks on the side Upper Level and Lower Level, about 1/2" apart. So if it's 1/2 to 1" lower than Lower Level, should I fill it to that line first? (This is an academic question this time. It's mostly charged already. I didn't notice the low water level before.)
Let me ask also if i need to buy my friend a replacement battery: I borrowed this one last fall, and charged it just enough to replace what I was using, to the best of my estimation, but when the weather turned cold, I didn't use it or charge it.
Since yesterday, it had been charging at just 1 amp for 18 hours and was just up to 12.6 volts, with the charger disconnected, when I noticed the water level was an inch below "Lower Level". Do you think I could have used up all that water in less than one day? Or was it low when I got it, or when I stopped using it last fall. Did I damage his battery?

Finding the camera, the cable, etc. would take me quite a while.
Did you look at the Amazon ad at the top. Especially if you put the cursor over it so it enlarges, you can see the top of the 6 little yellow caps/plugs/corks**. They're in a rectangular channel in the top and I can turn each a little bit with pliers and it comes out.
The 7th hole is in a second channel, just as big as the first, closer to the center of the battery, with only one plug, next to the 4th plug in the first channel. You can see the top of the yellow plug peaking out behind the black edge of the top of the battery. It's just as big as and just like the others and why not as much of it shows... It must be pushed in farther.
**They don't screw in like "caps" implies to me. And they're made out of plastic, not cork.
Looking at it more closely, shining a light in the hole beside it, I seem to see the light in the 7th hole too. Not so much the opposite way, but I'm starting to think it's two holes into the same cell. Why would they do that?
At first I thought maybe that one hole was meant for filling all the cells, and I thought all the cells were at the same low level. But then I tilted the battery, and of course the water did not all run to one end. Maybe one end of the cell, but not the battery. I also looked at all six and they really weren't at the same low level. So it's not meant to fill all six at the same time, but I don't know what it's for.
Trader wrote:

No. It has one of those too and it is at the end, with a clear ~6" hose attached.
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On 04/30/2016 08:07 PM, Micky wrote:

As long as the plates are covered when you are charging, you should be fine. Since your charger is only 1 amp, it's not likely you will "boil" the battery over.
The gassing begins at 2.37 v/ cell or 14.2v on charge total
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Thanks. It should be done by now, but it's downstairs. If I don't complain later, everything went fine. Once I'm done, I'll add more water almost to the Upper Level mark.
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On Saturday, April 30, 2016 at 6:22:54 PM UTC-4, Micky wrote:

Possibly for a vent/overflow hose? I've never seen one in the middle, typically they are the end.
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On 4/30/2016 6:22 PM, Micky wrote:

From what I see, I sense the 7th cap is for venting of electrolysis gasses.
On a golf course, the 19th hole is often for rehydration. Less so with garden tractor batteries.
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On Sun, 1 May 2016 13:45:45 -0400, Stormin Mormon

Well, I put a straw in the 7th hole and sucked out a Bloody Mary.
No, I still think the 7th hole went to the same place as the 4th hole. Why they had it , and why they had to put a second, rectangular channel in, is beyond me.
I returned the battery to my friend and it cranked is riding mower very well. If he'd had gasoline, it might even have started.
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wrote:

Possibly to get by a battery tie down?

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On Sun, 01 May 2016 21:05:34 -0500, "Dean Hoffman"

If you look at the picture, I don't think you'll think so.
I mentioned the 7th hole to my friend when I returned it to him, but he wasn't interested. He's a practical kind of guy. Theoretical quesitons don't interest him.

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On Monday, May 2, 2016 at 1:59:42 PM UTC-4, Micky wrote:

Ok, then judging from the size, maybe it's for your pecker.
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On Mon, 2 May 2016 11:26:39 -0700 (PDT), trader_4

Petty and vindictive, aren't you.
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Micky posted for all of us...

I can't see what you are talking about but it's either a mark made during manufacturing or like another poster stated for a vent hose.
You want us to answer the question but you are too lazy to get us a pix so... My crystal ball is cloudy.
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