I will bet someone greased that slip clutch to often. The 1660 I bought years ago would slip the clutch on the clean grain elevator when you got into heavy corn.
Opened up the clutch and found it full of grease.
Washed it up put it back together and never another issue.
Those zerks on slip clutches are to grease that shaft that the clutch spins on when it slips. Otherwise no grease is ever needed. So one pump of grease every 500 Hours or so is plenty for a slip clutch.