Owww i just recovered my old case from my mothers home last week.
Looky here :-)
But mine has got a metal lock/slot. I know it's old, i can remember having it like forever. At least somewhere from mid 80's.
I'm gonna try to remove the tape residue, my mom always has such great ideas
I do love her though!
edit:ooh my, second pic, right hand tray, bottom left: i suddenly see the part that was mentioned
here .