Type 15 Tail Fuze Model 1 and Model 2 - US Designations B-3(b) Type B-3(a) - used by the Japanese Imperial Navy during WWII.

The larger fuze (b) was used on heaver bombs (1000 -1800lb) and the smaller (a) used on bombs in the 500lb class. These both function in the same manner, but the larger fuze body was likely neccessary for heavier bombs to insure its function due to the greater disruptive force at impact.
(Shown above right is a 550lb bomb with a B-3(a) fuze installed. It was common practice to install nose and tail fuzes on a single bomb.)

In the unarmed condition, the striker assemply is locked in place by the wind vane which acts like a lock-nut on the threaded end of the shaft. The wind vane is also fixed by a removeable safety fork. A safety pin also locks the assmebly, as well as an internal shear wire. After loading the plane the safety pin and safety fork are removed. A small arm on the bomb rack replaces the fork to prevent the vanes from rotating in flight. When the bomb is released, the wind vane is free to rotate, unscrew and depart, unlocking the sleeve. At impact, inertia casues the arms to swing forward and with a camming action the striker is forced down, cutting the shear wire. The firing pin is then driven into primer on the gaine. The cam mechansim is an interesting dual function design which also drives the firing pin down when the arms are pulled up, possibly in the event of a water impact (?). This may explain why the arms have that currious cup feature at the ends.

An enlarged view of the B-3(b) fuze, showing the wind vane, safety fork and safety pin.
The red tag is an advisory tag with notes to the ground crew. There are variations from fuze to fuze, depending on specific functional details, but they all more or less state things like "Remove safety pin before plane leaves, put in the safety pin if they come back unused.". These would be left on as the plane was bombed up, and removed before flight, or re-attached if the ordnance was returned. The bright red color was a visual aid to quickly identify that the bombs were armed (or not).