Some notes on the colon-dash combination :—

FOR 

1876

The first long sound of each vowel is exemplified in the following words:—
John Ogilvie. The imperial dictionary. 1876 (originally published 1851)

1884

The  OED uses the combination :—, in a bold type, to indicate ‘normal development of’ in an etymology (1884). But this was a symbol, rather than a piece of punctuation.

1901–1912

The marked vowels are shown in the following line, which is printed at the top of each page:—
Chambers’s Twentieth Century Dictionary. Chambers, 1912 (originally published 1901)

1905–1933 (see 1956 in Against)

When necessary these have been indicated in the following way:—
F Howard Collins. Authors’ and printers’ dictionary. OUP, 7/e 1933 (originally published 1905)

1919

[concerning use of a dash]
With colon or other stop before a quotation.
11. Hear Milton:—How charming is divine Philosophy!
12. What says Bacon?—revenge is a kind of wild justice.
H W & F G Fowler. The king’s English.  OUP, 2/e 1919

1949 (see 1963 in Against)
Note the passive construction:—
     The servant was told to open the window.
A S Hornby. The advancer learner’s dictionary of current English. OUP, 1/e 1948

AGAINST 

1886

The vowels are as follows:
      a, like a in far or ask.
Charles Annandale. A concise dictionary of the English language. 1886

1951

The colon has a further purpose in directing attention forward (we have one golden rule: look before you leap), and there is much to be said for confining it to this. The dash — is sometimes used for the same purpose, but in the form :— it is superfluous and should be omitted
Brooke Crutchley. Preparation of manuscripts and correction of proofs.  CUP,  2/e 1964 (originally published 1951)

1954

With a colon to introduce a substantial quotation or a list (e.g. as follows:—). This, though common, is unnecessary since either the colon or the dash can do all that is needed by itself.
Sir Ernest Gowers. The complete plain words.  HMSO, 1/e 1954, transcribed from

1956 (see 1905–1933 in For)

When necessary these have been indicated in the following way:
F Howard Collins. Authors’ and printers’ dictionary. OUP, 10/e 1956

1963 (see 1949 in For)

Note the passive construction:
     1. He was elected king.
A S Hornby. The advancer learner’s dictionary of current English. OUP, 1/e 1948

1983 (but also much earlier!)

Omit the dash when the colon is used to preface a quotation or other matter, whether at the end of a break-line or not.
Hart’s rules for compositors and readers at the University Press, Oxford. 39/e 1983