Anyone know what this is?

[FarmGurl]

New to tractors here. I have a Kubota M108S tractor and there is a plug located next to the 3 point hitch at the back that kept popping out during use. Finally, I lost the cap the last time I was using it. Can anyone tell me what the plug is for? There is NOTHING in the manual at all. I don't know if it's a hydraulic plug or what? Do I need to be adding anything to that chamber? Any clarity on this will help. I've made a temporary plug for now until I understand more about this. Pictures to show where I am talking about and what the tractor is.

 

[Lil Foot]

Not familiar with your model, but my guess would be that that is an access point for an adjustment.
I close up pic down the hole from straight above might clarify that.
In the mean time, I would find something to cap that hole.
Doesn't look like a place you want dirt entering.
If you can't get a new replacement cap from Kubota, maybe something like this:

https://www.amazon.com/FMF-Racing-11299-Wash-Plug/dp/B0068C5OO6/ref=sr_1_6?dib=eyJ2IjoiMSJ9.8MeZq0LO0dRu057-ckEa-1uZIgIbCei0U9zcrn6ov3gyPvhZ7BNXvjvOaZvD-VB79RlMfOVDxTrAqxQH-YMgs9RgjblK0nQvI9MqybxudK9VzxKfFGv9XrGR_BHSIq-_CY8r9vOxgorJ6Ki27vFUWJ0xEffAwbAr1HO77RvYEKAkofegCxxUo4cuXozyC4HFyAXVA6M1cxbGWnwaxF-KJ1Wg6Ng8hlKs3FLex-MuMhEMrdNriJPux7-QS-fgHO6jftg-SC7ryd_M0w9TVdGFVf35DAm52lN3J4S1OrWtKac.0iZ7D3ebYWyTZDcUk11u1hg5AVu4sOMJP-AFuSRagb4&dib_tag=se&keywords=dirt+bike+exhaust+plug&qid=1746132577&sr=8-6

Just cut it off as needed.
BTDT

 

[Old Machinist]

Looks like an oil fill. My guess if it keeps popping out is you have a vent clogged somewhere.

 

[Mitjam]

New to tractors here. I have a Kubota M108S tractor and there is a plug located next to the 3 point hitch at the back that kept popping out during use. Finally, I lost the cap the last time I was using it. Can anyone tell me what the plug is for? There is NOTHING in the manual at all. I don't know if it's a hydraulic plug or what? Do I need to be adding anything to that chamber? Any clarity on this will help. I've made a temporary plug for now until I understand more about this. Pictures to show where I am talking about and what the tractor is.

I’m looking at my tractor right now. your missing a rubber plug. I would be ordering one it plugs off an inspection/ service hole for the transmission.

 

[Dustyx2]

I’m looking at my tractor right now. your missing a rubber plug. I would be ordering one it plugs off an inspection/ service hole for the transmission.

And check you vent. It's probably blocked and pressure build up is what was pushing it out.

 

[Mitjam]

New to tractors here. I have a Kubota M108S tractor and there is a plug located next to the 3 point hitch at the back that kept popping out during use. Finally, I lost the cap the last time I was using it. Can anyone tell me what the plug is for? There is NOTHING in the manual at all. I don't know if it's a hydraulic plug or what? Do I need to be adding anything to that chamber? Any clarity on this will help. I've made a temporary plug for now until I understand more about this. Pictures to show where I am talking about and what the tractor

And check you vent. It's probably blocked and pressure build up is what was pushing it out.

yes. This plug may be a vent plug too not just a normal plug

 

[D2Cat]

Maybe that rubber plug has a vent in it, and it got plugged by a mud dobber?

 

[North Idaho Wolfman]

This is the plug, put some RTV sealer on it when you install it.

The vent is up front: