I've no idea....I'm don't even know when they started using them. I've not seen anything definitive about them at all.
I've looked at the Kahr website using the Wayback Machine and it goes back to 1997. I couldn't find any photos of magazines until 1999 and those show the plastic follower with steel insert. I did see some review articles from the 1997 pages that appear to show the plastic follower, but the photos are really small.
My guess: either the metal follower was used in the first couple of years of Kahr production or they were a batch run done concurrent with the plastic follower (why would be anyone's guess). On a side note, I've only seen them in MK/PM/CM/Covert size flush fitting magazines.
Hopefully someone else knows the time line and can enlighten us.
Regards,
Greg