My information comes from "Ontario Broken Circles" by W. Bruce Graham. He lists 4 hammers used with ST CATHARINES WEST. I read the wrong column when I made my previous post. The given period of use is July 1869 to Feb 1884.
Note that there is some overlap (after 1875) with cancels that just read ST CATHARINES. Cancels used before 1869 (with one exception) read ST CATHERINES.
Thanks again, Jan. When I checked the gc site for post offices, they did not list St Catharines West, so I turned away from that as a possibility. I am a member of the PHSC, so have access to the data base that incorporates Bruce Graham's work. However, the data base doesn't agree with him on the period of use of the hammer you refer to. Based on my cover, it would appear that Mr. Graham is correct. I measure the diameter of the postmark as a definite 22mm. I don't know if the pen notation of the date is authentic. Cheers Ross