That designates high flow.
The jet would flow a hair more than rated but not quite up to the next size.
I don't know about the 3rd digit, but I'd just assume take the other guys word for it to no real consequence.
They've intro'd and discontinued many things over the years, like the anti siphon/anti pull over nozzels.. went from ball to valve pin under the nozzles at the same time...then no brass tubes...then brass tubes...economizer pv's.
How old is this carburetor?
