Over at the Mill they posted the first page of HB 367 from the 1921 Legislative session, which updated the 1919 gas tax.
|The 1921 update to gas tax|
via The Mill (yellow added)
About it they wrote:
History is in the Details. In February 1919, Oregon became the first State in the Nation to pass a per-gallon gas tax (one cent). As this bill shows, an additional one cent per gallon tax was brought up just two years later.
But they may have missed the more interesting detail.