This is an old usage, now obsolete. Graves and Hodge (The Reader Over Your Shoulder, 1943) describe it thus:
A long dash may be put after a colon, for emphasis. For example:
‘The Captain arose and said: “Come, Antonio, amuse the men, and tell them one of your favourite stories!” Antonio arose, rolled the quid from side to side in his coarse mouth and, after a pause, began thus:—
“About the year 1874, in Lisbon . . . ”’
Note that the colon-dash construction is distinct from the internal colon.
OED 1 employs :— in etymologies to signify an “extant representative, or regular phonetic descendant of”. According to tchrist, OED 2 and OED 3 employ it similarly to signify “normal development of”.