The reason a red seal $5 bill doesn't have a Federal Reserve letter on it is because the red seal version is NOT a Federal Reserve note. Red seal bills are United States notes.
"A" is the highest series letter for 1928 $50 Federal Reserve Notes. "K" is most likely the Federal Reserve District letter. The series letter, if any, on US bills is next to the date. Please see the question "What is the value of a 1928 US 50 dollar Federal Reserve Note?" for more information on values.
The large letter in the center of the deal indicates the Federal Reserve District.
All U.S. dollar bills have a letter A through L to indicate a Federal Reserve Bank branch where the bill was first issued. K refers to the Federal Reserve Bank of Dallas.
There were no series letters on any 1929 US bills. You may be referring instead to the Federal Reserve District letter; "G" indicates your bill was distributed by the Chicago district. There's more information at the question "What is the value of a 1929 US 100 dollar Federal Reserve Note?"
The 1934 series only extends to the letter D. You may be confusing the series letter with Federal Reserve Bank letter, which is "E" for the Richmond Federal Reserve District.There's more information at the Related Question.Note that the bill was actually printed in Washington; the Richmond district ordered it and distributed it.
Please post a new, separate question with the bill's date and what letter, if any, is next to the date. The Federal Reserve has been issuing $1.00 bills for almost a century.
"D" is the highest series letter for a 1934 $10 FRN. The series letter is next to the date. A "J" would most likely be a Federal Reserve District letter. Please see the Related Question for more information.
The "4-D" indicates you have an early Federal Reserve Note; those are the code number and letter for the Cleveland OH district. There's more information at the question "What is the value of a 1914 US 5 dollar Federal Reserve Note?".
Until recent redesigns of currency US bills that were "Federal Reserve Notes" (almost all bills printed) had a single large letter between A and L, which identified one of the twelve regional branches of the Federal Reserve Bank. (for example, notes with the letter "D" were issued from the Federal Reserve Bank in Cleveland, OH) New designs for most US bills omit these letters. They remain only on one-dollar bills.
Bills from the Kansas City Federal Reserve Bank will have the letter J and the number 10 on them.
$5 Federal Reserve Notes have been issued for over a century. Please check your bill again and post a new, separate question with its date and what letter if any is next to the date.
The Federal Reserve Bank of Los Angeles uses the letter code "L" and the numeric code "12". To clear things up, the bill wasn't actually printed by the Los Angeles Federal Reserve Bank. It was distributed through that bank but was printed in either Washington of Fort Worth at the request of the district.