The Mack RWL uses the 1891 bulb for both front and rear side marker positions across the 1981-1985 model years.
The 1891 is an ANSI-standardized bulb, meaning it conforms to a defined specification recognized across the United States and Canada.
The 1891 is a T5 miniature wedge bulb, a form factor characterized by a wedge-shaped base that seats directly into a socket without a separate metal base or pins.
The T5 designation refers to the bulb's tubular envelope, with the numeric portion indicating a width of five-eighths of an inch.
The 1891 operates at 12 volts, which is consistent with standard 12-volt automotive electrical systems.
At 2 watts, the 1891 draws very low current, a characteristic typical of side marker and indicator applications rather than primary lighting functions.
The use of a single bulb type across both front and rear side marker positions on the Mack RWL means one part number serves both locations, which simplifies parts sourcing.
Because the 1891 is an ANSI-standardized type, it is produced by multiple manufacturers, and any 1891-rated T5 wedge bulb meeting the 12V, 2W specification is a functional equivalent.