Tim, Procor cars wouldn’t have been visible until the 1960s… this Union Tank subsidiary wasn’t created until the mid-1940s as Products Tank Line of Canada. However, this company's cars were not visibly identified as such, bearing the same UTLX reporting marks and Union Tank Line labeling until the company was renamed to Procor Limited in the summer of 1962, at which point it seems that they were fairly aggressive in clearly promoting their new name. They continued to share the UTLX reporting marks until 1981, when the PROX reporting mark was used for new deliveries (older cars continued to use UTLX reporting marks until about 1990). The company did change the labeling to PROCOR LIMITED at that time, later shortened to just PROCOR.

Canadian General Transit dates to 1931, and has since their beginning been a subsidiary of General American Transportation. I can only assume that their cars were primarily in Canadian service, and therefore didn’t show up much south of the border — possibly GATX cars were more commonly used in cross-border service.