This sentence is from an English classic, Dr. Kane’s “Arctic Explorations.” The first comma and dash perform a double office: they indicate and set off the appositive group that follows, and also act as the dash and comma in the preceding example.
20. The same note of character—the craftsman’s keen delight in work—is struck in “Adam Bede” and in the little poem on Stradivarius.
Here is a fairly well-established conventional punctuation which we do not like. The relation of the group of words following the first dash to what precedes it, is that of apposition, and is best shown by the comma, with the dash for grouping. This use would require the repetition of the comma and dash after the group. The comma and dash would conform to the punctuation of the examples considered above.
21. We went into the trenches a full regiment. We came out to retreat again with four hundred men—and I left my younger brother there.
22. In spite of his harsh, stern exterior, the man had wonderful depths of emotion and nervous sensibility. I think you can see it in his face—when you have discovered it otherwise.
Much of the beauty of language is lost to one who cannot read into the dash in No. 21 a scene of tender emotion, and into the dash in No. 22 a bit of philosophy, as well as a bit of humor.