View source for Mould markings

LEGO factory moulds have markings that are imprinted on the LEGO elements that they produce. The most common type of mould markings are two numbers with a hyphen, as follows: X-YY. The X is usually the position of the element in the mould, and YY describes which mould iteration the part comes from. As an example, 1-02 means that it's the second mould of this element made (02) and the piece comes from the first cavity of the mould (1). 

With LEGO elements produced in large quantities, such as the 2x4 brick, it is common to see the mould markings use 3 digits, in which case the number always reflects the mould iteration.

Certain LEGO elements, especially small elements or some older LEGO elements, do not have mould markings at all, and some even lack the LEGO logo. It is in such cases very hard to tell what mould iteration the element comes from.