1918 Harley-Davidson TECH:

23985-18

The (-18) designates the first model year this part was utilized on a 

Harley-Davidson motorcycle . . .

If H-D made a change to this part after a few years . . . they may add a letter(A) to the end of the part number to indicate a change in the part.

Example: 23985-18A

If another change was made, they may add a (B)or(C)

Example 23985-18B  or maybe 23985-18C

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Many times, you can almost determine what model the part is for via part number.  The first year of the particular model is the end number of the part number.  This is a good thing to know when digging through swap meet NOS parts in NOS packages . . . This ain't written in stone by any means . . . and engine parts (especially things like keys above) are utilized on new 2016 models.  The same part used in the motor all those years.  It gives you a good idea what part you have, for what model. The 23985- of the part number also falls into it's own, special category . . . and it has taken me years to figure this out, which you'll just have to find out for yourself - I ain't got that much time to explain.

WL Parts: -36, -41, -46

K Model Parts:  -52, -54, -55, -56

Ironhead Sportster Parts -52, -54, -56, -57, -58, -71, -72

Panhead Parts -48, -49, -57, -58, -65

Knucklehead Parts -36, -41, -46, -47

Shovelhead Parts -66, -68, -70, -75, -79

FXR Parts -82, -83, -84, -86, -89, -92