Hi vinay,
I have a T51 (actually the MP4Nation customized high memory capacity version with 16GB of Flash Memory, called the s:flo2), have had it since the first batch of s:flo2... it is working like a champ!,
There was I believe the T50, T50+, T51 (aka s:flo2), and eventually T59 (in the future) for the T Series.
I'm not too sure about the X Series, but there's the X19, the X19+, and now in the new "concept" HIFI series, the X19HD... also the X30 (which I have on order, and it should be arriving in May, I'll review it. I needed something Fuze-ish with an TF/uSDHC card slot, and a more durable housing, the X30 is metal.).
I don't want to speak for anybody, but I believe RAFA is fairly familiar with Teclast's product lineup, correct me if I'm wrong on that RAFA and I hope I haven't spoken out of turn.
There's also their 4" and 5" screen PMP series players, Android Tablets, and good old fashioned MP3 players other than the ones I mentioned, but I've said all I am familiar with. Hopefully someone else can be more helpful.