Could be a good old fashion American fractional number.
We just did the wheel bearings on my son's 05 Durango and had to use a 1 3/8" socket from a 3/4" drive set to remove and install the front axle nuts. I've only used the sockets from that set a few times, and its pretty much been the same socket, and I have replaced the pinion seals on a 9 1/4 before. The reducer from the 3/4" drive down to a 1/2" drive is very robust, its been used with my 750 ft/lbs impact gun several times.

Can't say for sure, but I do have a late 70s 9 1/4 in my lower garage, and I'll be down there sometime this week to get the mower out. We went form 6" of snow to needing to mow parts of my front yard in 2 days!