It is typically called a J hook and it inserted in the oblong key way in the front frame rail.
Bob,
The picture you included has the caption of "t-hook", which is what we've always called them.
Jim
I am aware of the other name. There is also a R hook. J hook is what I have always heard them generically referred to as is why I choose to continue calling it that . In conversations with others over the years that is what they are referred to as so apparently it is a accepted generic alternative name in more then one circle. You can call them whatever you want if it bothers your sensibilities. Online they are often listed as a J hook cluster which includes the several different versions...
Attached is a picture of a typical "RTJ Cluster" used on towing trucks and cargo trailers.
Personally, having driven tow trucks... many names are indeed used, and often based upon geographic areas as well (think "dialect") I am sure...
... in my neck of the woods, the "J Hook" is a really large J-shaped hook that could hook around an axle housing and usually on its own chain or paired with another large J-shaped hook on a sling with a center ring.(to hook twice to the forwards or rearwards tie-down).
In towing, I usually didn't use any other hook except this large J-hook ot sometimes, what I called a T-hook (either one or the other).
This cluster pictured represents IMHO, each "letter-shaped hook" found on what at least this one SELLER calls an RTJ Cluster. Personal choice, I am sure but I will call the hooks by their "letter shape".