Yup, maximum fill indicator - offset to indicate maxiumum level if filled on the prop stand. Need to leave some expansion space in the tank, or the expansion of the cold fuel due to absorbing engine heat leads to expensive & volatile puddles under the overflow pipe (if it has one) or fuel running out of the filler cap if it doesn't.
The small diameter petrol nozzles usually fit past them, though ? Mrs B's Moto Guzzi V7 Special has a plate with a hole that the nozzle only just fits into + a couple of small diameter vent holes. This is a pig to fill, as you can't see what is happening as you fill. I guess the idea is to stop splash back. Though as the V7 has a 22 litre tank & the Interceptor is only 13.5 litre, even though the Interceptor does around 10% better mpg it always needs filling before the Guzzi, so not great problem if the Guzzi isn't "brimmed".