I've got photos of two of those cars, Charlie. They're attached.

I did a little digging, and it looks like these were BUILT by the CB&Q, but directly for Swift. The cars first appear in the SLSE ORER listings around 1926, which is when the Q was building the SM-18 class stock cars. (SM-18: no end doors, built by CB&Q 1926-1928. SM-16: with end doors, built ACF 1922 and CB&Q 1924). The cars always ran under Swift branding (SLSE, SLST, SLSX), but for three different owners: Swift, which sold their car line to Quaker City in 1930, and then to GATX which bought Quaker City in 1934. There was lots of that sort of private car shuffling going on in the early years of the Depression.