Didn't the NPbuild a few bay window jobs out of their boxcars? The roof profile is what made me think that, nut the orange says Milw or ??
One internet source says Butte Anaconda & Pacific #4, however with this pic of that caboose as marked, the details conflict with your pic as being #4: the vertical grabs next to the door are short in the "as marked" pic, and are longer on yours and positioned further out, the orientation of the smoke stack is 90 degrees off - either detail could be changed over time, but looking at the end platforms they're vastly different in shape from the steps and whatever that horizontal area is called above the coupler. The round markers and general color and style support that it could be a BAP caboose - they seemed to have had a mixed bag of caboose styles.
It would appear to be BA&P #12: http://www.rrpicturearchives.net/showPicture.aspx?id=1734549 https://photos.smugmug.com/Trains/Historic-Railroading-Outside/i-LXjdtsc/0/07ee6380/O/IMG_1823.jpg
Met don when I lived over that way. He lived in a wide vision BN/NP caboose at the time. He sure had a large collection of stuff!