The first three numbers of the part number denote the vehicle that the part was first used on. If it is continued on late vehicles it will not be renumbered. As a general rule it would work like this. 111 would tell you that it was for the 25-36hp Type I vehicle or engine. 113 for the 40hp Type I vehicle or engine, 131 for some 1967 and later vehicles, 151 Type I convertible vehicle, 141/143 KG convertible vehicle, 311 for the Type III 50hp engine and vehicle, 211 for the Type II 50hp engine and vehicle and 411 for Type IV engine and vehicle and 1972 and later bus engine. We now have numbers such as 040, 041, 025, and 039 for other applications.
The center three numbers tells where the part fits on the vehicle. All numbers in the 100-199 are engine parts. 200-299 fuel and muffler parts. 300-399 are transmission parts. 400-499 are front axle related. 500-599 are rear axle related, 600-699 wheels brakes related, 700-799 bumper, cables and shift related parts, 800-899 body interior parts, 900-999 are electrical.
Finally the last three and any trailing letter or number is the actual part number.
"The reason I have listed this information is to provide people with at least a little guide as to the part number system for ordering parts. We make every effort to list the proper description of parts, however, you would not order 113 parts when we list a 311 part for your Type III vehicle."