Model 4 Groove cover with knurled nuts were fitted to The Automatic and Model 4 std from 110,000- to approx 180,000 as original examples have been seen this high. They stopped using the knurled nuts when they ran out.
The Cover with clip started with the Model 5 SS1 155,000-Unknown (Somewhere in the model 5 S2 run). No model 5 got the knurled nuts, even thoug the Model 4 and 5 intersected.
So as Gadge says, no one knows exactly, but I do know they were NOT fitted to a portion of the Series 2 run. So apart from spares allocation and the Series 3 plain base due for release, they simply just used them till they ran out, leaving a portion of them without the cover.
If I had to give an amount, going of evidence, id say it was not long past the halfway mark on the Model 5 S2.
