First Alphabet Rate Change Stamp
On May 22, 1978, the US issued its first in a long series of Alphabet rate change stamps.
In the years prior, other stamps had been issued non-denominated for impending rate changes. Because postage rates for late 1975 were uncertain, both of the Christmas stamps that year (#1579 and #1580) were issued without denominations. These were the first non-denominated US stamps.