Henry Adams’s use of punctuation, “:—” [duplicate]

Taking a break from typing court orders, and specifically

The Court has sent sealed copies of this Order to: -

Smith & Jones

Brown & Robinson

and the Court of Appeal

I can say that it is used in formal contexts as a strengthened colon: that is, either at the beginning of a list (as in my example), or before an expansion of what has just been said, as in Steve Melnikoff's example. I imagine it is a shortening of viz:- and was later abbreviated to a simple colon.

I haven't seen that used before. Reading your example sentence, I treat it just like a regular —. You can probably do whatever you want in your own books, but I would just use —.