MLB Baseball: The Senior League?

On the TV show MASH, in an episode where they bet who will win the 1951 World Series, the term “Senior League” is used to describe the league the Dodgers are in, as I recall. What does that mean? Googling, I can only find this refers to a softball league? What’s the SD on this?

The National League is older than the American League, and is sometimes referred to as the senior league.

The NL was founded in 1876, the AL in 1901.

I believe the exact term is “Senior Circuit”.

Yes, the phrase was a staple of old-timey sports writing, but was invariably written (and spoken) as “senior circuit”. (And “junior circuit”.)