The 1891 bulb is the sole bulb type listed across all side marker positions on the Mack F for the 1981-1982 model years.
The 1891 is classified as a miniature wedge bulb, using a T5 form factor, which refers to a tubular glass envelope approximately 5/8 of an inch wide.
The wedge base design means the bulb is held in place by the socket gripping the base of the glass directly, with no separate metal base or pins involved.
The 1891 operates at 12 volts and draws 2 watts, placing it among the lower-wattage bulbs used in automotive marker lighting applications.
The 1891 carries ANSI standardization recognized in both the United States and Canada, meaning its electrical and physical specifications conform to a defined North American standard.
The Mack F uses the same 1891 bulb for both the front and rear side marker positions across the 1981-1982 model years, indicating no distinction in bulb specification between front and rear fitment.
Because the 1891 follows an ANSI standard, replacement bulbs from different manufacturers carrying the same designation are expected to share identical voltage, wattage, and base type.
The T5 wedge form factor is a compact design commonly found in low-intensity marker and indicator applications rather than primary illumination roles.