1950s Repro Vintage Sewing Pattern: Open Back Dress. Butterick 5605
'Half size' is a figure type, like 'womens', 'misses', or 'juniors'. Half size patterns are designed to fit best on a someone with a fully developed figure and shorter back-waist length. Waist and hips are larger in proportion to her bust than in the other figure types.
Half size patterns can be identified by a ½ in the size number (for example, size 16½ instead of size 16) and the actual bust/waist/hip measurements will be odd numbers (for example, bust 39")
Unprinted / Precut
During the early 20th century, sewing patterns were sold with just a single size in the envelope and the pattern pieces themselves were plain pieces of tissue, already cut to size, with no printed markings. Notches and other important points of note were marked by punched holes instead of ink.
Individual pattern pieces are identified by either a letter, number, or full pattern name punched out of the tissue.
Some people are intimidated by the idea of unprinted pattern pieces but they're really very similar to use (and in some ways more convenient because you can make your tailors tack or chalk markings directly through the punched holes).