It's signature because it comes with a very long strap and you can only play in "one, two, three, four, five, six, seven, eight!"-time with it.
It's nice that The Ramones (or their descendants) are at least seeing now some money.
And I don't believe for a minute that Johnny couldn't afford a seedy gig bag for his Mosrite or whatever it was - I believe the paper bag was a statement of protest against other musos and legend building ahead of its time.
My son would never put his guitar in a gig bag or case either when he went to rehearsals (he had both, in good quality). His argument was "that we attract more attention with wearing the guitars open". So his poor (Custom Shop!) Les Paul was dragged through the London Tube while he lived there with the guys from his then band. And it shows to this day!