1987-D Dime mintmark location

coinvac41

While searching rolls of dimes I noticed the mintmark is located in 3 different locations on some 1987-D coins. Is there an online site where I could research this and see if there is anything significant here or just different die sets? If push comes to shove is there a book that describes all of the Roosevelt series?

MWallace

At that time mint marks were put on each individual working die by hand so slight variations of location is normal.