How does this thing weigh for a 240z?

Less than the alternator...

According to my precise? bathroom scale it weighs 12 lbs including the pulley, mounting brackets and two hoses. Since mine resides on a shelf it was easy to pull out and weigh

I have never seen one.

I believe all 240s came with "smog" (air) pumps from the factory, but it was usually one of the first things removed in a belief it would somehow improve performance. The only ones that stayed operational were from the emission testing areas. Some people just removed the drive belt thinking it would be simple to restore the function when necessary, but then exhaust gases would backup into the system and destroy the inside of all the valves and lock up the pump.