It's Snoopy's first time doing the "Happiness" dance, here with forelegs folded in a Russian style. It's also Snoopy's first time as the life of the party.
It's not his first time with a thought balloon. If I'm remembering right, it is the third legitimate time his thoughts have been represented. One of the two times was with the now-familiar thought bubble (with small circles replacing a tail), and the other time was like it is here, with a tail on the balloon. It is also the second time Snoopy's doghouse has been depicted with a TV antenna.
This is an important strip along Snoopy's development. Except for the way it is drawn, it could easily be a strip from ten years later. It is solidly Classic, as opposed to Early, Peanuts.
As far as the question of Snoopy's ownership, this is another step away from his being owned by Charlie Brown or another kid, back towards his being a neighborhood dog who's just "around," although he does seem to own his own house. (And a TV set and electrical power.)
