The shorter #505 is for 6-wheel trucks and the longer #508 is for 4-wheel trucks. Of course, there are exceptions like the 72-foot Con-Cor passenger cars that have their 4-wheel trucks mounted closer to the end of the car than usual and therefore use the #505 instead of the normally used #508. Also, there have been some 6-wheel trucks being used as 4-wheel trucks simply by removing the middle set of wheels.
Passenger cars can be equipped with truck-mounted couplers using the correct conversion bolster attachment. Truck-mounted couplers permit closer coupling on radii down to 18" without causing a binding for most passenger cars. Non-Magnetic trucks should be used with Kadee® Magne-Matic® Couplers. Wheels should be changed on AHM and Athearn trucks. Use Kadee® #521 or #522 non-magnetic wheels.
Each kit contains a pair of Draft Gear Boxes, one pair of NO.5® Couplers, #634 Centering Springs, screws, special bolsters, and instructions.