I'm not an expert on the C22, but I've owned three Catalinas.
The fuel locker change happened in the late '70s for the C25; since it was a huge safety issue, I can't imagine that the C22 didn't get the same fix at about the same time.
In the mid '80s every Catalina went from MkI to MkII. Well, most of them. I know the C22, C25 & C30 did. The C18 was never a MkI, it was born with the MkII features.
The MkII includes (but is not limited to):
More angular shape to the lights, no rounded corners
No aluminum trim rings around the lights
A more aggressive non-skid pattern (little pyramids instead of a criss-cross pattern)
Life lines with 2 rows of wire (maybe not all boats)
That should be enough for you to recognize a MkII vs. a MkI. There were many more changes to the individual boats, some of them significant.
Don't be afraid of a MkI; they were (and some still are) fine boats. Catalina Direct still has tons of parts for fixes and upgrades. In fact, CD is one of the big reasons to buy a Catalina.
If you can wait a year, there will be a C25 for sale close to me.
Very close to me.