WARNING! The following is off-topic -- but worth thinking about.

These dummy cells are supposed to be energy savers, because they
let you use only three AA alkaline cells in a four-cell flash. This works,
because most flashes operate correctly with nicad or NiMH cells, which
are nominally 1.25V (rather than the 1.5V of alkalines).

However, this is a great way to waste money -- and not just the cost
of the dummy cell. With only three cells in the flash, you won't be able
to discharge them to as low a voltage as you would with four. That means
you'll have to discard them sooner, and you'll get less total energy out
of
them.

I just called Quantum, and found out that these actually aren't "energy
savers" at all. In fact, there might be circuitry in them (which is why
there are two models).

If you own a Canon flash (I assume Nikons are the same) and have read the
wretched manual, you know that, when using an external power pack, you must
leave the AA cells in the flash, because they continue to power the control
circuitry. It appears that going from 4 cells to 3 reduces the drain on the
cells, and presumably extends their life.