Can anyone identify this piece?

[cba191]

I just got an Octo Classic 202-s and a Reefmat 500. I found this and am not sure where it goes.

 

[paraletho]

Can’t say as I have used neither. Is that like an O Ring by your thumb.

 

[cba191]

It's all rubber

 

[paraletho]

It almost looks like it could be a drain plug for the bottom of the skimmer cup.

 

[paraletho]

Or one of those rubber mounting/ noise dampening feet under a pump or motor

 

[cba191]

I checked both those spots and those are all there.

 

[TangerineSpeedo]

Or one of those rubber mounting/ noise dampening feet under a pump or motor

Since it has some calcified bits on it probably this