Putting together a Multicolor 500 Refrigerator Storage Set one piece at a time? They're not all the same. Older pieces have a beveled top edge that later ones do not. They're even a fraction of an inch taller. Besides the obvious (once you know it) physical difference, they can also be told apart by mold numbers, which are otherwise not usually significant. The older dish on the left has a mold number prefixed "A-". Later dishes are prefixed "B-" or have no mold number prefix. Color, in the case of the 501s, also corresponds to the difference in the shades of red of the early vs. later 402s. Since clear versions of 501s and 502s made from both molds exist, we may safely assume the change took place in the very late 1940s to early 1950s, when those items were sold.