Literally where the word "sinister" comes from. It used to mean left-handed.

I had a Great Uncle that was a lefty and he was forced to write with his right hand. He had the most beautiful cursive penmanship you ever saw. So flowing and elegant. It was perfect. I can't imagine his struggle to be that good.

Both my parents were lefties also, (what are those odds?). And I was the only one of the us 4 kids to be so afflicted. And in the early 1960s when I learned to write, I drove my teachers to the brink of madness being a lefty. I was constantly admonished to keep my paper properly vertical and not turned 90 degrees. And my letters never had the correct slant to them, in cursive the letters need to slant forward just a bit. Mine tended to slant backwards. I also often got lower marks on my papers because of my left handed penmanship. And to be fair, my penmanship is still poor to this day. Writing in the western world is not made for us members of the Bar Sinister.

As far as Ned owning the Leftorum and being right handed, that is the joke within the joke.

It's a Catholic school thing but they've been somewhat pushed out of education in the 60s's quiet revolution.

This was common in France too.

My wife's mom was left handed, but forced to write using her right hand (in the Netherlands in the 1950s).

It actually helped her when she got some degenerative nerve disease since her right hand was the last thing to lose control, so she could write even when she couldn't speak. 😟

My grandmother had said when she was learning to write, they would tie her left hand behind her back to force her to use her right.

This is pretty much it. Like I learned everything using my right hand. Turns out I can play guitar and write with my left hand, and type faster than most of the older people I've met with my left hand alone.