We'll start with an overview of the cluster of buildings along the tracks here. Motor Parts Distributors was the curving building at the far end.
Zooming in on that final building reveals some ornate concrete details along the top and clearly displays the curved nature of the structure.
I captured some closeup shots of the painted signage on the building.
After studying the track diagram, the Sanborn Map and the GIS data, and the following modern aerial photo, the property also consists of two structures on the upper level closer to Terrace St.
The northernmost (upper) building visible from above is a brick affair built into a just above the hillside.
The Quonset hut structure between the two is apparently connected to the second floor of the lower building and was likely used for some amount of shipping or receiving. Note the "MPD" lettering that's still on the building to the right of the door.
The Frisco industry diagram shows active rail spots at both the lower doors on the main brick structure as well as the upper dock on the Quonset hut structure (separately listed as spot #336-1).
Unfortunately, since this company was dissolved prior to the dawn of the internet, there's very little information available online. I was able to track a couple of links to the business owner (RICHARD F. MILLARD, JR) and saw that they were specifically hiring a woman (!) in 1974 for general office work, but that's about the extent of it.
Perhaps further research can be conducted at the local library. Until then, I'll have to go with conjecture and assume that various carloads of auto parts were received at this location for local distribution. Considering that having "distributor" in it's name and based on the relatively small size of the facility it is quite unlikely to have been involved in the production of automotive components, though there are major plants in the metro area for both Ford (Claycomo) and GM (Fairfax).