I asked the same question at Yahoo! Japan Chiebukuro (a Japanese Q&A site).
The answerer replied,
This would be an old holder for a fishing rod, whose spike is impeded into the soil while fishermen are fishing.
Then I asked him if it had been used on the ground, and he replied,
after considering the longitude of this stuff, this might also be used as a horizontal holder for a fishing rod, it may be placed in front of a ship instead (so that you can place the fishing rod on the U-type prongs.)
After googling in Japanese "fishing rod, Meiji era, holder", nothing such old came up but a today's holder, which has also the U-type prong.
I am sorry that I am unable to present an old picture of the fishing rod holder, but I think mine would be probably correct since,
For paddle or hook, U-type prong won't work properly.
For fishing gaff, the U-type prong also doesn't serve as a gaff because it is not sharp enough.