What is this yellow component? A battery?

I'm tearing down a radio for fun. I found this component. What is this exactly? It looks like a battery to me but it seems odd as it requires 3 AA batteries, why would their be another one inside?

I just realized that this hand radio (it was not mine), has a mechanism where you can turn a handle, this would make sense if this was a rechargeable battery. I am getting amount 50mV across it. I'm guessing it's dead. What do you guys think?

Perhaps a capacitor, or a supercapacitor (high capacitance value). Do you have a multimeter that can take capacitance readings?

Looks like electrolytic capacitor. Does it have any other marking?

It is presumably, a supercapacitor.

Let's see the other side!

If its a hand-cranked radio its definitely a dual-layer capacitor, a.k.a. supercapacitor, ultracapacitor.

It has to charge quickly, store a fair amount of energy, and survive total discharge - batteries cannot
handle over-discharge, it wrecks them very quickly, electrolytics have very low energy density,
therefore its a supercap. A battery would have a terminal at each end too...

batteries cannot handle over-discharge, it wrecks them very quickly

That depends on the chemistry. I had some old (maybe 10 years old) barely used 9V NiMh batteries, I charged them up and did a discharge test and got the rated capacity.

NiMH are definitely hardier than some, and discharge in storage may be rather different in chemisty, but in general
over-discharge is an expensive mistake...