Unbelievable!
I've been around cars all my life and owned at least one (and in most cases a lot more) of just about every early (pre Commodore) Holden made, and I've never paid attention to what side the oil filler cap was on. I must have a dozen sets of standard Holden V8 rocker covers in the shed and had no idea that they were different.

I do usually fit alloy or chrome after-market ones, so that may be the reason.

But the information network on this forum is staggering....... you're never too old to learn something new.