A few things. You had a very rare after market glass Amperite ballast
That's only the second 303 glass ballast I've seen in many years of looking.
They do not have a shield around them. They are prized because they actually regulate the current to the tubes. They're filled with hydrogen gas and have finely twisted iron wires. The hydrogen seeps into the iron and affects it's resistance. When the wire get's hot, some of the hydrogen is forced out and the resistance drops. That causes the wire to cool and it absorbs more gas.
That perforated metal tube is the stock ballast. It's just some resistance wire on a mica wafer like a toaster. That metal shield is to keep fingers away.
Here are four types. Left to right.
Clarostat aftermarket replacement.
Stock Motorola ballast.
Amperite glass ballast (this is a more common '459 type)
Homebrew using Capacitors, resistors and thermistor.
