R4 Compressor Disassembly for housing seal changeout?

[FL Chevy]

The R4 compressor on my 91 Chevy full size truck is leaking around the O ring seals of the outer housing. The compressor is a 1yr old unit I bought new, not a rebuild. It's possible it may have been overcharged, but not sure.
I would like to replace the O ring seals. Thought I would practice on the old original R4 I had laying around before I attempt the seal change out on the good compressor.
Question is this: How do I get the outer housing off to get to the O rings?
So far I'm unable to get the housing off on my 15year old practice core. Is there a trick?
I do have a clutch removal tool if complete disassembly is required.
Thanks for advice!!

[GM Tech]

The outer shell is pressed on over the two big o-rings- pull the right angle tab back- and use another old shell to place over the top and bridge it with a good piece of oak two by four and give it one good wack with a 10 pound hammer and it pops right off- then I sand down all the sealing surfaces and clean them and re-oil them and put the shell back on and line up the other side of the shell and put the oak bridge across it and one good wack and the shell is back on...pretty easy if you have an old shell......already off the compressor.......