I installed the Mor/Ryde 1621 \5 on my Gateway ,the 3,4,5,or 6 is the weight of the rig . Mine was the 14,000 to 18,000 weight
i am very happy with mine.
Here is a model chart ,I found mine on Amazon for a great price...$430.00 plus free shipping with Prime ,I found a returned one for 60% off...