Early 1920 Holley HN Typical 1921-23 Holley NH 1925-26 Holley NH. The 1923-25 version of this model was identical except for the needle valve adjuster assembly. Introduced in 1920, the first version of the NH had an almost straight-through passage from intake to outlet (left). Shortly after this a revised version with a dip in this passage became standard. There were several modifications in the NH; early models (until about 1923) had the bowl drain valve off-center, separate from the bowl-retaining screw.
In 1923 the carburetor was modified and then the drain was part of the retaining screw at the bottom. In 1925 the needle valve adjuster was changed to the universal-joint type (late 1925 and 1926 cars). This carburetor was manufactured by both Holley and Ford; the Ford version being labeled with the Ford name.