Hi Boeing767mech, Buzz here. I don't think UAL had any of the "C" model DC-7's, I could be wrong though. The DC-7C had an extended wing center section, more fuel there.
Dad was going through flight engineer school in early '66 and there were only a few DC-7's for training, the DC-6B was still used for short haul operations..... in the pre-737 days. Caravelles were for East Coast stuff.
Idea: look on United's web site, there's a section called "Photo Gallery" and shots of each of what UAL has flown over the years. You might see what you're looking for there.
The DC-6's were just plain aluminum color downstream of the engines, and the R-2800 engines had aluminum cowlings.
The DC-7 had R-3350 engines (50% bigger), leaked a lot more oil, and had aluminum cowlings with Titainium nacelles. So that might be what you're noticing.
Some operations painted the area downstream of the engines gloss black, to disguise the oil mess that radial engines make. We measure oil consumption in gallons per hour on radials, modern engines use tenths-of-quarts per hour. It's a different way of life.
Buzz Fuselsausage, Line Mechanic by night, DC-3 Crew Chief by choice.