The caboose is most likely DM&IR 187. the D&IR had a few of these long (for that time period) cabooses built in 1887. At that time they were longer than the standard 4-wheel caboose being used on the railroad. Their orginal numbers would have been D&IR 87 and 89. After the merger of the D&IR and DM&N into the DM&IR in 1927 they were renumbered by placing a one in front of their number – eg #87 became #187, etc. At that time they were give class number K-3. D&IR did not use class numbers for their freight equipment, but since the DM&N did and they were running the show, they were provided class numbers. At some time (I think in the 1940’s) the DM&IR changed their reporting marks (the letters used on their freight cars to identify the railroad) from DM&IR to DMIR.
The Missabe Railroad Historical Society has a web page where they list the DM&IR cabooses. http://www.missabe.com/caboose.html
They list K-3 cabooses 181, 184, 185, 187, 188 and 189. They have photos of 184, 187 and 189.
The photos show a small window on the end shown on the left of the photos of 184 and 189, but not on 187. There is not such a window on the caboose at Pine River. So based on the limited photos above, I would guess it may be caboose 187.
#184 was retired in 1947. The caboose you have was in use as late as 1960.
When built brick red – very dark red with gold leaf lettering
1920’s mineral red with white lettering
1937 yellow with black lettering. In the 1950’s lettering changed to vermillion on some equipment.