Marc, the valve that seals the gas in is the one in the cylinder you are filling. Make sure you unscrew that from the brass adapter when you have filled. Do not unscrew the adapter from the extinguisher first as this will vent all the gas you have filled. As Harvey says you can weigh the cylinder empty before filling. To get the most C02 in put the cylinder in your freezer but not for too long as you can freeze up the valve in it. The warmer the dispenser (extinguisher) and the colder the receiver (cylinder) you will transfer more CO2.It is not necessary to fill the cylinder completely to the specific weight mentioned, you will still get a lot of shots. On your photos you show a brass fitting, has that got the female extinguisher thread cut into it ? If you were in UK I would have liked to buy it from you or swap a nice target pistol such as the Baikal IZH 46M.

Baz.