If you're asking how this is possible or how it is implemented, it's necessary to type a "high space" before these characters, because they obviously kern off the lefthand edge of the type body. If that wasn't your question or you already knew that, sorry.

Thank you for the answer; this is my question: how characters above 18 units were handled.

I'm surprised, though, that this was done so... routinely. I would have thought that the characters would have been scaled to fit 18 units (not by changing their general size - a higher set would be used) so as to avoid anything as cumbersome as that. Even several sizes of Caslon Old Face, I've found, had this property.

Also: for trying to help, even if you had gotten my question wrong, I would still owe you thanks, and you would not owe me an apology.

You're very welcome. I learned this working on a Monotype caster this past summer. They also have two different keys for a, e, and o: one set sets the letters with a negative left kern, for using after T, V, etc. A lot to remember. So to type "Wa", you had to type a high space, then W, and then remember to use the alternate "a" key. But yes, it is strange that you need to use a high space so frequently. I think the reason they couldn't just make the character bodies wider is that the lead would take too long to cool and mess up the caster somehow. This is why I was told that you can't type em/em/em/em etc to fill out a blank line. You need to type en/en/en/en because having too many wide characters in a row causes problems as the metal cools.

Josh