When you want to get a collectible piece you need to know whether the item is factory  first or second quality, as deemed by the factories at the time of production.

Royal Copenhagen has 3 wavy horizontal lines as part of their mark. If the lines have a vertical line or "slash" through it, it is a factory 2nd, if not it is a factory 1st.

Bing & Grondahl has 3 towers as part of their mark. If the towers have a horizontal line or "slash" through them, it is a factory 2nd, if not it is a factory 1st.