Modifications made to the highlights in the hair were made and the series was re-issued as the 1954 "Modified" Series.
Modifications to the face plates were made for most denominations in 1956, except the $1000 bill, which was modified some years later.
This series lasted until the early 1970's and were finally replaced when the alphabet was used up in the serial numbers. Counterfeiting had
also become a serious problem.
If you're looking for scarce versions of these bills, look for the final signature sets as they were produced only in small quantities and
then replaced by the next series.