Steve Sheflin has posted a query over on the Technical Forum regarding this/these mystery aircraft; I found the following five pictures of the aircraft in question on Ebay several months ago, and am posting them here for further discussion.

A few interesting points to note; the upper left TS+MB has the short tail, while that in the third pic has a tall tail; the upper left one also appears to have a deeper oil cooler than normal for a G-6 (Fo987 maybe?); and the Stkz. lettering is slightly closer together on the third pic than the first.

The obvious initial conclusion would be that this is some sort of unit identifier... there's no conclusive evidence to support this, however.  Maybe these were the only stencils available to the ground crew to paint Stammkennzeichen?  :)

Of course, if anyone has further details they can add, they would be most welcome!