This is a handheld pong. The biggest paddle includes the system hardware. The second one wich is smaller (1/3 of the system size) is attached to the first one but can be detached. Thus, this is a real portable and easy to carry pong system.
The 666s and 666t seem to be similar in every ways apart from their color: the 666s is blue while the 666t is green.
Note that there is a french version of the 666s called the 666s-p (P for PAL ?) while the 666s-n is the english version. Both are blue and absolutely similar, except for the writings which are in french on the french model...
The model 677 is the same system with two shooting games.