from Fancyclopedia 2 ca. 1959 and Fancyclopedia 1 ca. 1944
Perhaps nowhere have fans shown more varied ingenuity than in simulating brackets with only the resources of the typerkeyboard. Some, 'tis true, draw them in afterwards [like Swisher] but most fans use the keys, which is more convenient. Tucker and others employ double parentheses a half space apart ((apart)), while Speer sometimes uses -(Gregg shorthand parentheses)-. Youd's /brackets with underlining/ made with the virgule have been the most popular, tho the underlining is frequently dispensed with — or mutated; Mirta Forsto used tildes for underlining. Eney and some others use the /crossed slant-bar/ .

The purpose of brackets is to distinguish ordinary parentheses by the writer from editorial comments such as [nuts! -ed] inserted in the body of a letter or article. Ray Bradbury tried an unsuccessful mutation in simply making his comments ALL-CAP.

from Fancyclopedia 2 Supplement ca. 1960:
I forgot to mention the commonest method of all: double parentheses a full space apart ( ( ) ). /That's one of the drawbacks of chasing the esoteric: I miss the blatantly obvious./ Some get so enamored of double parens that they use 'em even when not needed; Larry Stark was a conspicuous sinner.