The Burner 1 stage was an Altair rocket stage as used for the third stage of some Vanguard launch vehicles, but equipped by Boeing with 3-axis control.
This combination was used for six vehicles. The first was launched 1965-01-18 and the sixth 1966-03-30. These were early launches of classified Defense Meteorological Satellite Program satellites. One of these launches failed.
The Burner 2 used with the Thor-Burner was the first solid fuel upper-stage vehicle used for general space applications that had full control and guidance capability. The first Burner II flight was on 1966-09-15.