Retired Reading Company Conductor George Ether sold modified K-Line cabooses under the grandstand at the York Meet. I always enjoyed talking with him. Here is a photo of his illuminated Reading caboose with pairs of windows and the K-Line box. It was along time ago, but I vaguely recall replacing K-Line trucks with those diecast trucks.
Williams acquired AMT / KMT tooling and sold cars and cabooses as kits. This may be one.
The dates on the bottom - 1989, 1990, and 1991 - are important clues. Back then, MPC / Lionel, Weaver, and Williams trains were tapping a growing market. But catalogs were more like sales flyers. Nothing like today's vast variety of trains was available. The only 3-rail scale trains were reissues of Lionel's Hudson and Pennsy B6 switcher as well as Williams Crown Edition Hudsons, NYC Niagaras, N&W J's and SP Cab-Forwards. Readily available parts enabled lots of kitbashing to fill gaps in rosters.
IMHO, that caboose is a perfect example of a widespread aspect of 3-Rail "O"Gauge railroading back then. So many trains are available today that kitbashing has declined considerably, though examples are posted on the Forum. But around 1990 (and earlier), lots of people did it.
Even the green Madison cars by Madison Hardware in another Topic are an offshoot of that. Lionel heavyweights were last cataloged in 1950. Take some bodies, frames, trucks (and lights?) in stock for years; paint them; letter them; assemble them; sell them to fill gaps in customers' rosters.
Please send photos and a brief article to Ed Boyle for Collector's Gallery on OGR. That caboose and those dates tell an important story.