The threading is ISO metric "Acme," M16x4. I would guess that the female thread is the full M16, however, the threading on the towing eye is slightly undersized at 15.5mm, with the shoulders of the teeth rounded off, probably to make sure it can be threaded in and removed easily by hand.
Acme thread is not a fastener thread like UFN, UTS, etc. that binds due to friction between the threads. Acme is most commonly used to translate rotational motion to linear motion (think bench vise, screw jack, "C" clamp, lead screws on lathes). By nature an Acme thread is free moving and will not bind, and why, I'm sure, it was chosen for the towing eyes. Acme thread is also very strong when stressed linearly (again, towing).
The Acme's characteristics may not make for a solid fastening that will not loosen. Even my very light GoMINIGo front license plate bracket vibrated loose once.