Anyone know what this is and who makes it?
 

Anyone know what and who?

trick question or are you legitimately interested in what it is?

Got any pics with clearer resolution that show any markings?

If I had to guess, it might be a NiMh pack of cells for supplying power to the Vcc

I really would like to know!

Like Bill said, it could be a small battery for power.

I had another idea, because it is kind of small for any useful power source I know of. It could just be a capacitor to reduce high current draw from a servo from drawing receiver voltage too low.

Here's how to test; get out your multi-meter.
1st) Use voltage setting and measure voltage at plug. It could be dead, but even a fraction of a volt would tell you that it is not a passive device.
2nd) After 1 proves there is no voltage present. Use the ohm setting and check if charge builds across it, proving that it is a capacitor. You will notice that the ohm reading will continue to go up.

Hope that helps.