Starter plunger cap O ring

I have a 77 KZ650 that I took the carbs off and cleaned, put in a carb kit,
got new starter plungers and O rings. I had an O ring in the carb kit that
I thought was for the starter plunger cap, (it goes between the cap and
the carb body) but it was slightly too big.
The O rings I got with the starter plungers were the same, slightly too big.

The old O rings fit down inside the body of the carb, in what looks like
an indentation. But the O ring doesn't look quite as thick as the indentation
from eyeballing it. I would assume the O ring would be slightly thicker
to make a seal. But they're old too, and may be compressed by now.

The new O rings are just big enough they don't fit down into the indentation.
If I try, the O ring sits on the carb body, not in the indentation. So I
can't get the cap screwed down flush to the carb body.

So, is the cap supposed to screw down flush to the carb body?
Or do you tighten it down snug with the O ring between the cap and body?
If those aren't the right O rings, any ideas on how to put it back together?

Thanks, hopefully after the snow tomorrow (Ohio) it will be time to ride.