[jogosub]

Curt, Reese, U-haul and many others are all made by the same company: Cequent.
I have U-haul 2" and it's perfect. For less than $ 200 it's much cheper than Subaru OEM; that hitch is the small square ( 1,5" I believe) while all aftermarkets mentioned above are 2" square hitches. Google U-Haul and you will see it there!

 

[jogosub]

OK - sorry about that. Then, I don't have an answer because I have U-Haul 2" hitch and I use different pins for different applications. For my bike carrier I use Yakima's pin/bolt that has a thread and you have to screw it into the carrier. Most bike carriers have this design - the hitch/pin is threaded!
For other applications I have a lockable pin (with no thread) that fits exactly the hole that is in the receiver.
Best is to take a caliper and measure the diameter as you have already suggested or use the pin, that came with the bike carrier! Those, typically, do not need to be "lockable" because one needs a wrench to take them out.