US Stamp 599 and 599A are both 2 cents carmin Washington 1923 issue. Both are Vertical coil with Perforation 10.
Stamp 599 is Type I (worth $.15 used and $.50 Mint Value) and stamp 599A is Type II variety (worth $17.60 used and $133.45 Mint Value) as of 2010 catalog prices. As you can see it is important to tell these two stamps apart. Otherwise you will be overpaying many folds.
Type II has 3 heavy hair lines at top center of Washington's head. 2 of the hairlines are very outstanding in the white area (for the lack of better words- the bald area) of Washington's head.
This stamp would have been easy to observe if the USPS postal mark had not gone right through the white area of washington's head. To see the feature save the images and enlarge it in your picture viewer of choice.