[PhasmaFelis]

I'm not sure how you could get more obvious than a picture of an Alt key and a picture of a Left Arrow key.

[sp332]

Backspace is often represented by a left arrow. https://www.bing.com/images/search?q=backspace In context, it could have been confusing.

[DonHopkins]

What does an "alt" key even look like? Is it the one labeled "option"?

http://www.cultofmac.com/3147/opinion-apple-keyboards-need-b...

https://en.wikipedia.org/wiki/Option_key

[PhasmaFelis]

It hardly matters, since the Mac shortcut for back is Command-LeftArrow.