It's indeed the detonator of a torpedo.
But would the u-boat number be stamped on the detonator, when not even the u-boat itself was allowed to carry a number for identification? Well, I don't know, it's of course possible, but I would think that the numbers rather may represent the type of detonator mechanism or "pistol"
This one could for example be a further development or subtype of the Pi 36 pistol that was developed in 1938 and onwards.

I have checked with Eberhard Rössler's Die Torpedos der deutschen U-Boote ISBB 3-8132-0842-7
I don't find any picture that exactly matches this detonator, but close.

The four large fins or horns are the contact triggers that will move on a hinge upon contact with a ships hull and trigger the detonation of the initial explosive (firing charge) which then in turn would ignite the main explosive of the torpedo.
The four small fins is part of a security mechanism. They make the tip into a propeller which will rotate when the torpedo moves through the water. This rotations will screw an internal cylinder further into the head. The cylinder contains the firing charge, and only after the torpedo has run a long enough distance (the tip has had enough rotations), the firing charge has been moved into the right location where it can be set off. This is of course to ensure that the torpedo doesn't explode too close to the u-boat.

In page 78 of the book, there is a picture of a Pi 1 (Pi G7H) head/pistol. This one only has two “propeller” blades, but he mentions that later in development, they used four.

Cheers
Vidar