Yup, that is a standard size store "display case", pre 1960's at least. Don't think 40'ish though, but maybe. They normally had 2 shelves in them, a narrow 1 and a wide 1, that each rested on 3 moveable brackets that were inserted into "slotted" metal strips that were affixed to the 3 upright braces of the back side (2 corner, 1 center). Yours is easily worth $100+. Antique shops, pawn shops, single proprietor shops, etc. are always needing display cases.