Jisho

One guess I had was that it's a reference to the fact that this kanji is an old version that now gets written as 缶 in modern Japanese.

缶 is the "left side" of 罐.

But it would be kind of weird to represent that information that way, so I have my doubts about whether this is actually the reason or not.

That was one of my thoughts, but I don't know either. What's frustrating is if you google the phrase "steam boiler (left side only)" you can find 200 pages in various dictionary sites that just quote it verbatim as if it means something, seems like no one has stopped to think if it makes any sense!
I came across the kanji in Murakami's latest novel, where he uses it to spell 薬罐 (kettle), which is a "steam boiler".