Tappet Weight

The LEFT tappet is lighter by about 1/2 oz. or almost 1 oz. total valve train weight(X 4ea) ...at 5000 rpms, that's a difference...more weight for the valve spring to push down - to hold the tappet on the cam...

1953 Harley KK Model (and some KRs) had hollow tappet adjusters too,

with a hole right down the middle.
 

NOTE: The RIGHT tappet is stock in Harley WL engines, and what makes it so heavy is that it's thicker, and solid steel. The LEFT tappet is standard on early 900 Sportster engines.

You can run the lighter tappets in WLs, K Models, whatever...they're dimensionally interchangeable, same length, diameter, etc.  To most folks, ounces only matter at the post office, or your drug dealer, or the judge.