IMNA trademark L, but basically what Levi's appears to have is protection against others using their stylized 501 logo in commerce. Obviously, if you put their logo on a 501.tld name and tried to sell jeans you would have problems.
That is what every blooming TM is about, letters or numbers.
That is like saying a phrase, word, or number can not be TM'd.
Thankfully, you just closed down a massive government bureaucratic machine and money drain if what you say is true.
Yes, numbers can be TM'd. Levi has a TM on the number 501.
There are 2,984 live TM's associated with the number 3, 16 for 501, 104 for 401.
That is what that copy and paste was about.
Obviously, if you put their logo on a 501.tld name and tried to sell jeans you would have problems.
You would not even need to put their logo up.
All you would have to do is put 501.tld and have any jeans appear and risk WIPO and lawsuit.