The Packard Six was a series of luxury automobiles built over several generations by Packard from 1913 until 1947. The name was originally used to describe the car in general terms, while Series numbers were initially used and changed every year to denote wheelbases, then the number classification changed as market conditions changed so as to keep competitive with other luxury brands.
There are three generations that used a six-cylinder engine before World War II, with varying engine displacements and periodic mechanical changes.
Second Generation (1921–1928)
Beginning in 1921, the L-head engine was once again the only engine offered with a 116" wheelbase. It was officially identified as the Packard Single Six until it was joined with the Packard Single Eight in 1924. Once the eight cylinder engine was introduced, the Six was repositioned as a mid-level luxury car competing with the Buick Master Six and later the Chrysler Six, with a retail price of a 1921 5-passenger sedan listed at US$4,940 ($81,049 in 2022 dollars).
This generation introduced updated vehicle identification: 1st digit is series (Six only, Twin Sixes and Eights had their own series designation), 2nd and 3rd digits refer to wheelbase. F.e.: 233 is 2nd series Six, (longer) 133" wheelbase. 326 is 3rd series Six, (shorter) 126" wheelbase.
The line consisted of:
1st Series (Single Six)
series 116, 116 in. wheelbase (1921-1922). This model was also designated the Single Six.
series 126, 126 in. wheelbase (1922-1923). This model was also designated the Single Six.
series 133, 133 in. wheelbase (1922-1923). This model was also designated the Single Six.
2nd Series (Six)
series 226, 126 in. wheelbase
series 233, 133 in. wheelbase
3rd Series (Six)
series 326, 126 in. wheelbase
series 333, 133 in. wheelbase
4th Series (Six)
series 426, 126 in. wheelbase
series 433, 133 in. wheelbase
5th Series (Six)
series 526, 126 in. wheelbase
series 533, 133 in. wheelbase
The Six was discontinued after the 5th series, and there was no Packard six cylinder car until the 1937 One-Ten. As the complicated naming system was revised for 1929, 626 and 633 refer to the new 1929 Packard Standard Eight in a similar way. 5th series Eight was omitted.
Source: Wikipedia