Griswold #7 Large Block EPU with HR
Learning how to identify cast cast iron is an important part of collecting. Knowing your logos will help you know when to spend the money, or when to walk away. When I first started collecting, all I looked for was the Griswold name, not knowing that different logos are indicative of when a piece was made, and it's value.
On a Griswold, 'EPU' stands for Erie, PA, USA, which was where it was made. 'HR' stands for heat ring. Where the difficulty arises with identifying vintage cast iron is with pieces with no logos. Most can be identified, but then it's a question of knowing your handles, skillet shapes, pour spouts, and other markings that can be used to identify a piece.
If you want to get into collecting, learning how to identify is a must. A great and trusted resource for this is castironcollector.com.