What were—or are—"odd pants?"

Solution 1:

odd pants, in that they were not part of a suit (coat pants with or without vest)

Growing Up: Childhood in English Canada from the Great War to the Age of Television (Themes in Canadian Social History)Aug 16, 1997 by Neil Sutherland google books

"For Hugh Palmer's 'Sunday best,' his father 'bought me a pair of black Oxfords to go with my blue serge suit. How I hated that suit. The short trousers, or "stovepipes," were unlined, and the serge was a particularly rough sort - no doubt designed to cause maximum chafing just above a boy's knees.'11 In Cedar Cottage, one man explained, boys' Sunday outfits 'weren't suits, you know ... odd pants and some gloves, a shirt or something ... but they weren't too fancy clothes at that..."

It appears that after the great war, clothing factories sprung up specializing in just [odd] pants, to the chagrin of complete suit manufacturers.

"One of the most popular styles of the odd pants was the very loose white flannel trousers. They were worn by men who were extremely class conscious and wanted to visually remove themselves from the less financially fortunate. In The Great Gatsby, Nick describes that for the lawn party he “dressed up in white flannels....” The two other kinds of odd pants worn by men were the trousers called Oxford Bags and knickers." Google books: Fashion in the Time of the Great Gatsby

And now ... just pants. Whereas men would own many suits, they now own a suit or 2, 5 or so sport jackets and numerous pants with odd and varied patterns, colors etc. And ... definitely a number of pair of khakis.

Solution 2:

Fashion in the Time of the Great Gatsby by LaLonnie Lehman indicates that "odd clothing" was a trend in the development of more casual fashions that arose in the 1920s of having suits consisting of "pants, coats, and vests that did not all match in color and fabric." Your example is from a few years earlier, and you say you've come across the term in ads going back to 1850, so I don't know if earlier uses of the phrase meant something else, or whether the the trend actually wasn't new--perhaps it was a revival--in the 1920s but Lehman was unaware of that.

What I can only surmise, between your ad and my source, is that these were pants of the type worn with vests and jackets, but sold on their own rather than as part of a matched set.