The Mack CF uses the 1891 bulb across all listed model years, 1981-1991, for both the front and rear side marker positions.
The 1891 is classified as a miniature wedge bulb under the T5 designation, which refers to its tubular glass envelope size of 5/8 inch in width.
The 1891 operates at 12 volts and draws 2 watts, placing it among the lowest-wattage bulbs in automotive lighting applications.
As an ANSI-standardized bulb, the 1891 conforms to specifications recognized in both the United States and Canada.
The wedge base design of the 1891 means the bulb is held in its socket by the tension of the base itself, with no separate metal bayonet pins or screw threads involved in retention.
The front and rear side marker positions on the Mack CF share the same bulb specification, meaning a single bulb type serves both locations across the full 1981-1991 model range.