Have you tried measuring it Jasmin? Often when you get those early numeric tags they're a reference to the bust size, eg 46 inches.
There are many sizing systems from different places and eras, and you also have to consider that garments have often been altered so the size label won't necessarily be accurate any way, so I always size on measurements.
For example, say the bust on a '20s dress was 46 inches, I'd expect the waist to be about 46 inches too and the hips 48 inches. According to the size charts that I've drawn up, that would a modern size 20 Australian/16 US.