From inside (document excerpt):
Form No. 3325-516 Super Recyclerr Mower Model No. 20042210000001 and Up Model No. 20043210000001 and Up Parts Catalog Ordering Replacement Parts To order replacement parts, please supply: the part number, the quantity, and the description of each part desired. Understanding Reference Numbers Each identified part in an illustration has a reference number. The reference number for a part also appears in the parts list, along with other information about the part. This catalog uses two special reference number formats, one to indicate parts in a service assembly and another to indicate the quantity of a given part in an illustration.
Service Assembly Reference Numbers Parts in service assemblies have reference numbers in the form a:b. The a represents the reference number of the entire service assembly and the b represents a sequential number unique to each part within the service assembly. For example, a wheel assembly might be identified by reference number 6, the tire by 6:1, the valve by 6:2, and the wheel by 6:3. When you order the assembly identified by reference number 6, you receive all parts identified by reference numbers 6:1, 6:2, and 6:3. However, you may also order any part individually. Reference numbers of this type appear in illustrations and in part lists. Reference Numbers Indicating Quantity In an illustration, if a reference number indicates more than one part, the reference number has the form nX y.