The Mack U uses the 1891 bulb in both the front and rear side marker positions across the 1981-1989 model years.
The 1891 is an ANSI-standardized bulb, meaning its specifications are governed by a recognized North American standard applicable in both the United States and Canada.
The 1891 bulb follows the T5 form factor, a miniature wedge-base type. The T5 designation refers to a tubular envelope with a width of 5/8 inch, and the wedge base eliminates the need for a separate socket with pins, as the glass envelope itself forms the electrical contact.
The 1891 operates at 12 volts and draws 2 watts, placing it among the lower-output bulbs used in automotive marker lighting, consistent with its role as a visibility indicator rather than an illumination source.
The identical bulb specification across both the front and rear side marker positions means a single bulb type services all side marker sockets on the vehicle.
The wedge base of the 1891 is not interchangeable with bayonet-base or screw-base miniature bulbs, even when voltage and wattage ratings are similar, due to the physical differences in base construction.