It's the kind of small ring that comes with the chainset, installed on the inside where it'd usually be.
3/32 chainring, 1/8 chain and sprocket. Chain line is slightly off, but no problem, there's a little float with the chain being wider than the chainring.
Hiya mate, probably an amateur question but what chainring did you use on your Ultegra to make it single speed? Assume it’s 1 1/8th?