It's a metric O ring, LR part #: ERR6434
You should be more than able to match this up at any NAPA or Fastenal, etc.
Specs:
ID 19 mm
OD 23 mm
Thick 2.5 mm

 

Or that.

 

Thank you! It looks like some kind of sealant was used before.

 

I think you are right re: the RTV.
It always drives me crazy when people HALFA$$ a repair (just to get home from on the trail not withstanding).
The stupid O ring only costs $2.00 or less online.
At least you know to repair this right it will be very simple and cheap.