Hi War Eagle,
For the second stamp, the corner letters or check letters are the position on the sheet. For this issue printed in sheets of 480 - 24 stamps Across and 20 stamps down. The check letters run from AA to XT and AA to TX.
You can also check the plate of this stamp by looking next to the value about where the line cuts 1/2 there will be a number - 1,2,3,4,5,6,8,9,10,11,12,13,14,15,19 or 20. Plate 9 being the rarest plate.
Further information on this site
http://www.members.tripod.com/~pennyreds/