Hey Larry,

The 3906 is the model that most refer to as "the old model". The way I easily identify that model is by looking at the chassis. It has a black chassis, and the chassis has lots of vents in it: http://t1.gstatic.com/images?q=tbn:A...Zz2wMq4Ty0C1Hf

From what I've gathered, the 3905 and 3903 are basically the same truck except for the transmitter and receiver. The 3903 model was upgraded with the newer 2.4ghz technology. You can easily identify either one by looking at the transmitter. The 3905 has a metal, retractable antenna, where the 3903 has a short black antenna that's not retractable. 3905: http://static.rcgroups.net/forums/at...?d=1262742563; 3903: http://www.parkzone.com/ProdInfo/TRA...RA2209-450.jpg

The 3908 has some differences from the 3905 and 3903. The largest difference is that the 3908 has a brushless power system (motor and esc). It also has different wheels, tires, 17mm wheel hubs, which are made of aluminum and anodized blue in color from Traxxas. The 3908 also has white shock springs, where the 3903 and 3905 have red springs (however, someone could have easily upgraded to different shocks so I wouldn't judge the truck by that feature alone). I have also noticed that the 3908 chassis has mounting points for the brushless esc fan cover, whereas my 3905 does not. The 3908 also has black chrome front and rear bumpers.

Hope this helps,

Casey