|There were different numbers for early K-models, but by the time 1958 rolled around, the second year of the Sportster, this gear has stayed the same,other than a -58A in 1979 to 1984.
I am pretty sure it will retrofit to earlier years. Note the BOM table in the countershaft sub-assembly page.
Thing is, the -52 and -52A part has 22 teeth, the -54B, 54C, 58, and 58A have 21 teeth, so you have to retrofit the mate as well.
This page will be updated later with what to look for as this part wears or fails.
The left countershaft has a spiral gear as part of the low or 1st gear. It drives the speedometer on 1952 to 1972 models. The right countershaft is what would be used on 1973 to 1984 Iron Sportsters, the speedo is from the front wheel.
2nd gear is not next to 1st, or "low gear" as Harley calls it. It is not the second gear on the shaft but the gear that is transmitting power when you are in second gear.