The old cabooses were stripped to their frame and a new steel body was added.  This particular caboose was built as #604 sometime between July 1967 and April 1969, most likely from the frame of wooden caboose X604 which had been used for transfer runs around Green Bay.  

A window was centered on one side of the car body; the other side had a window offset to the right side.  Both had all-weather windows installed to help the conductor during cold Wisconsin winters.  Caboose #604 was renumbered to #104 in the winter of 1978-79 by simply repainting over the "6" on the car sides.  The photo of this caboose that I'm aware of was taken in September 1980 so presumably the caboose was taken of the roster around that time.  The slide you see below was developed in May 1980, so it shows the caboose near the end of its life on the GB&W.

An interesting side note in this photos is the boxcar behind the caboose.  Although it is lettered ROCK ISLAND, in March 1980 the assets of the Rock Island was sold in bankruptcy court and the Chicago & North Western acquired over three thousand Rock Island freight cars at this time -- note the small C&NW herald just to the right of the "K" in ROCK and notice the CNW reporting marks on the end of the car.  Ex-RI C&NW cars were a very common sight in Green Bay during the summer of 1980.