Purchased yet another Leica , a Leica IIF. The seller when asked ventured an "old case" might be around. When the beauty did arrive it was protected by a "looks new to me" case. The mint IIf and the mint-looking case also came with another find! inside the case I found a built-in slot. Inside the slot is a "won't come out" red card. Guess its been in there since what 1952(?), I learned long ago not to force when it comes to delicate things. So what is this thing?