I just spent a half hour nerding out on this with the old model-number-linked digital catalog system on collectingsnapon.com (yay!).
It's a tappet wrench, made between 1939 and 1955 for "late" Chrysler and DeSoto. They also made a CT-12 with a 3/8" opening for 1937 Chrysler and DeSoto, and an LT-17 with a 17/32" opening for Willys and International.
These SOE "Special Tappet Wrenches" were not always listed with the SOE "Long Tappet Wrenches" (LT-16, -18, and -20) on the tappet wrench page in every catalog in that timeframe, but sometimes in the back with miscellaneous special tools.
Not sure why they thought they had to make the CT-'s thicker than the LT-'s (Long Tappets) or the DOE ST's (Supreme Tappets) - which were advertised as "slenderized to the nth degree", but it was deliberate.
And if they were going to do that, also not sure why they gave them a different model numbering scheme from the other SOE "Heavy Duty Tappet Wrenches", which also had thicker shanks and heads and had LTX ('Long Tappet Xtra heavy duty', if I had to guess) series numbers through 1945 and XL- (Xtra Long?) series numbers after that. (
@MadeinCanada posted a snippet on page 6, post #238.)