OK, perhaps I can help dig this discussion out of the muck...
Dr_Sharp did help with some valuable info about this subject. I've purchased the hooks he recommended for my car because the OE Subaru parts appeared significantly more sturdy than the aftermarket blingy "racy" things found on Ebay, etc.
Here's why:
Number 1, it's all good quality zinc plated steel, made (probably forged) in 1 piece without a bunch of weak points built into the design.
Starting @ the threaded end,
M16 X 1.5 rolled thread; 32mm of threads that taper up to the 16mm O.D. This unthreaded section is 15mm long.
From there, it tapers up to the 20mm shaft O.D. over 6mm. The 20mm O.D. is 105mm long. The tow hook's total length is 204 mm.
These details all provide higher strength by eliminating stress risers; rolled threads are stronger than cut threads and tapered transitions don't cause stress-risers like stepped transitions.
The actual eye looks forged; it looks like some attempt was made to balance it's strength to the 16mm threads.
Almost every aftermarket versions had sharp; cut threads & immediately stepped up to the larger diameter. They all "featured" a much smaller than 16mm thread on the hook side, threaded into some part with something like an 8mm cross pin (guessing size). They all seemed to have some piece of U-shaped aluminum fit to that cross pin that looked like a pathetic attempt at a D-ring shackle.
A chain will break at its' weakest link, and you don't have to be much of an engineer to figure out what that link is in this case.
OK, back to the Subaru tow hooks on a Fiat 500.
When I test fit this on my car, I found that there is, indeed, nothing to thread into. It is a nice, precise fit between the hole and the tow hook.
So, at least with my car, I'm getting some beefy 16X1.5mm nuts and locknuts as well as a beefy washer to back it up. If others would like, I'll verify fitment in a few days.
These parts are on order from Mcmaster-Carr, the best hardware store on the planet.

I've
never, over 30+ years of doing business with them, been disappointed with the quality of anything.
9/16" | 0.625" | 1.375" | 0.110"-0.140" | Rockwell B68 | __ | 5 | 91525A339 |
For those of us without threaded holes in our bumpers, I guess we'll have to leave the tow hook in as a permanent feature. However - I'm willing to guarantee that the tow hook won't fail by stripping the threads on the nuts...
Some may notice that this is getting a bit expensive. However - it's a LOT sturdier AND cheaper than the "racy" ones that I've found.