gear box magnetic plug?

Greetings,

I recently acquired an even older 912UL than I had already.  The one I just got is 4005342, from about Summer of 1992 I believe.  I noticed that the gear box has what appears to be a magnetic plug, but on the newer (yet still really old) engine 4405028 (2002) there is just a flat spot in the casting that wasn't machined. 

What's the story with the extra plug?  Did they decide it wasn't needed later, or does it designate that the gear box is different in some other way?  This is 90% for curiosity, and 10% to make sure it's safe to remove it to peek inside.

Thanks,

Rusty (antique 912 collector)