I have these 2 Red Seal $2 bills. Why does 1 of them have just a C in one corner and the other corner has C 9? The other $2 has F 3 in both corners. Would this be an error?
No, it is not an error. The 1953C note has a position letter in the upper left corner and a position letter and plate number in the lower right corner. These notes were printed 12 to a sheet in two columns of six. The position letters started with A in the upper left and went down to F in the first column. G-L were in the second column. So the sheet looked like this: A G B H C I D J E K F L The 1963A notes were printed 32 to a sheet. The upper left letter and number together denote the position on the sheet based a grid that I cannot easily reproduce here but you can find at http://www.uspapermoney.info/general/note.html The fact that your note has F3 in both corners is coincidence. It was printed with plate 3 and is in position F3. None of the other notes on that sheet would have F3 in the upper left corner. Three others (positions F1, F2 and F4) would have their respective position number and letter in the upper left and F3 in the lower right.
On the 1953C note, the C in the upper left is the plate position letter. On 12 subject sheets (which this note came from) there was no number portion of the position letter. On later notes produced on 32 subject sheets (like the 1963A note) the position identifier consists of a letter and a number. The number in the lower right is the plate number. Go back and read the information in the link at post #2 for greater detail.
Here is another Series 1953 C $2 United States Note: Plate Position: A Front Plate Number: 15 Back Plate Number: 409
They had to go to quadrant system of letters and numbers with a 32 subject sheet because we only have 26 letters in the alphabet. Before that they only needed to use letters in the upper left.