you are thinking of the R134 cans, they have a threaded connection to attach a hose to.
the R12 cans use a clamp system that attaches to the outside of the top of the can, and a valve threads down to the metal you are describing. then you turn in the valve handle, and pierce the metal. then open the valve handle and charge the system
DO NOT REMOVE THE THIN METAL!!!

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!