The 1969-D FS-501 Quarter is one of two varieties recognized by collectors as being worth a premium. On this variety, the mintmark was first punched a little to far to the left and it was slanted slightly to the right. On the other variety (FS-502), the mintmark was punched to the left but it is was in the correct vertical position.
The small certified population for the 1969-D FS-501 Quarter is more a reflection of their low value than their rarity. The best examples appear at the MS66 level.