The LEFT tappet weighs about -5oz less . . .
X 4 each = 20 oz !
...or over 1 lb. extra valve train weight !
1953 Harley KK Model 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.