Google thinks the 303A20 part number belongs to a Rogers or CCI 0.03uF capacitor in a package like a 20-pin DIP. I couldn't find any pictures of it, though. It looks like the part coding is <capacitor code>A<package code>: i.e. a 104 (100nF) in a DIP40-like package is a 104A40. No idea what the A stands for.
The numbers I found could just be total coincidence, though. I've never seen one in my life.
It looks like a capacitor which is intended to be installed UNDER a DIL IC package.
The active connections are pins 7 and 14 (or 8 and 16), the GND and Vcc connections of the average 74 series IC.
The idea of mounting the capacitor under the IC is to minimise the lead length and improve the high frequency decoupling of the supply lines.
After abusing the one on my desk, it's made of plastic, with two metal plates inside. This pretty much confirms the capacitor theory. When I find out who borrowed my RCL meter, I'll confirm that.
you should have had the chance to look inside an early wireless C-Net telephone made by Motorola. There were hundreds of ICs all equipped with that kind of decoupling capacitors. There is one thing interesting to know about those too: the ground terminal was arranged on the upper part for extra ground shielding of the ICs.