This renumbering was caused by the assignment of numbers from 600 up to the new secondary system, but all three-digit numbers were affected.
At the same time, all numbers that conflicted with U.S.
Prior to 1933, routes were assigned by district.
Two-digit routes generally crossed district lines, while three-digit routes were assigned with their first digit as the district number.
The new system also grouped routes by district, but not as strictly (these routes could cross lines) and with no room for expansion; thus additional routes, starting later in 1933, often received numbers from 283 up.