They came out starter models 12D and 12K which covered models from.
Model 3,5,6,8,9,12.Utility models 501,509,510,521,522,540,550.Utility 125cc models 69VG & 69VG-1-2.Pace models 12A,69PB,69PB-1-2.
So as you can see there were quite a few models that had them installed.They were installed to keep debris from getting inside and obstructing inside there as well as I guess wondering fingers that may have wanted to get in there.I guess over time with things going wrong and people taking them off to fix something and or changing to a pulley stater they just seem to be harder to get now.
So good luck finding one Daryl you should be able to they are out there.
Here is a picture of one hope this can help.
[Linked Image]

Here for a good time,not a long time.