An earlier photo examined differences observed in the molds for 500 series Pyrex multicolor refrigerator dishes. The apparently older versions were slightly taller, with beveled top rims. This difference is also seen in clear versions of the 501s (and presumably the 502s as well) marketed in the very late 1940s to early 1950s. Since the clear 501s and 502s were absent from the 1953 Pyrex catalog, we might safely assume the change occurred before that time.