The three piece push rod did not have enough clearance, would rub and generate metal filings, and it would fall apart. The oil was supposed to travel down the center of the rod to oil the clutch.
The one piece is steel and a much smaller diameter. The oil flows on the outside. It doesn't fall apart or rub.
I've never heard of a plastic plug, the early ones had brass with no hole, then about early 2005 KTM put in an orifice. These need to be cleaned every 20k or so.
"Dad, can I get a motorcycle when I grow up?"
"Son, you can't do both"