All 2-135 and 2-155 tractors had the shifter on the side console, so that is not the deciding factor here. The series 3 tractors had the modular cab, and that is the easiest way to tell them apart. The quickest way to tell is if it has a separate lever for the park lock, then it is a series 1 or 2. Series 3 tractors had the park lock activated by the main shift lever. It useed gated rails to shift all of the gears, plus the park lock. 

Another place to look is the top fuel tank. If it tucks into the front of the dash and cab, it is a series 1 or 2, if there is a gap between the tank and the cab firewall, it is a Series 3.