Kevbo: Are you referring to cars/trucks or house locks? I am pretty dang old (maybe not the oldest contributer, but right up there). I don’t remeber any keys being used teeth or ridges down instead of up any application as the rule. I recall feeling the teeth with my thumb while fumbling to open a door in the dark and the teeth were always up. Some doors and ignitions in cars were used with teeth down, I think Chevys were that way, but I am a Ford guy and the teeth were usually always up. Thank goodnesss they are now reversable, very handy.
Now that I think about it, maybe it has to do (with house locks) as to how they were installed or the brand. Another example: In my house all of the electrical receptacles have the ground plug opening on the bottom where in our church, all of the ground holes are at the top of the plug-in.
I have another Inponderable that really bugs me, to follow later.