The Columbia Book of Yarn for 1915 contains this pattern for a crocheted spencer, here a sleeveless jacket rather longer than the usual waist- or below-bust length of a spencer. This version is for a "36 or 38 bust," as are many if not most of the items in this book -- for which we are grateful to the author, Anna Schumacker! -- and is worked in period 4-fold (4-ply) Germantown yarn, which Kim Salazar of "String or Nothing" tells us is roughly equivalent to a modern worsted, e.g. Cascade 220.
A year later, Corticelli revised the pattern* to use their Sweater Silk crochet thread or alternatively, their Princess Pearl Crochet Cotton --
Note that the Corticelli version is to fit a 34-inch bust, with the suggestion to add more chain stitches to the beginning chain for a larger size, "allowing about six chain stitches to an inch."
Princess Pearl Crochet Cotton seems to have been approximately equivalent to a modern size 8 crochet cotton.
*Yes, despite it being copyrighted by Columbia's publisher. The Corticelli version rewrote the pattern with their own abbreviations, but otherwise it is the same, even to the (presumably inadvertent) omission of instructions to work the left front.