My guess is similar to some other's guesses ^^^^^.
Just speculation:
The thin slot is probably for twisting, bending, sheet metal?, and the hammer end + the short handle probably was used for light knocking/adjusting-by-tapping (metal). I think roofing-tar used to come in metal buckets that had earlike-tabs around the circumference of the lid that had to be bent up before the lids could be removed from the top of the tar bucket. The hammer end of this old tool could be used to tap/bang the tar-bucket lids shut, and the slot-end used for re-bending down those same lid-tabs, for a secure closing.
Other than purely guessing, I have no clue. But, by the looks of that thing, it was made and probably used exclusively, for doing a dedicated, very specific task.