|There were different numbers for K-models, but by the time 1958, the second year of the Sportster, this gear has stayed the same.
I am pretty sure it will retrofit to earlier years. Note the BOM table in the countershaft sub-assembly page.
Thing is, the -52 part has 29 teeth, and the -52A and -58 gears have 27, 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.
Drive gear is next to 2nd gear. It drives the countershaft from the clutch gear, its mate on the mainshaft.