The plug-in thing looks like an air-flow meter or air temp meter to me, but I am not familiar with that engine. The 7.3 is a V8 diesel, yea? or gas? I thought Ford was using their own 6.0 diesels in 2003, so gas? Then air-flow or temp meter is a definite possibility. If so, it should plug into the side of the air-intake track between the filter and intake-manifold. However, an AF meter is vital to the motor running, so air-intake-temp-gauge is my final bet. I got $2 on it.
The black box between the air-filter housing and the turbo is likely a "reservoir." When you suddenly go from idle or cruising at low throttle to "petal-to-the-metal," or even a 20% sudden bump up, there is a momentary dramatic pressure drop in the intake tube, before the air molecules get up to speed and start pulling a greater mass of air through the filter. In a gas motor, at the same moment, your fuel delivery system (carb or FI) should be dumping a little extra bit of fuel into the motor, and if the air intake system lags behind even 0.01 seconds, the mixture will run very rich, and the motor may hiccup or seem to "lag" for a very brief moment. Alternately and additionally, it may change the "resonance" of the intake pulse (the peak-moments when an intake valve is open - the greater number of pistons, the weaker but faster the pulse), to yield both a smoother flow of air, and a quieter motor. So the mystery black box gives you better throttle response; and helps the 20,000+ (and growing) American children a year who get hospitalized from asthma attacks - at least that is what I remember blogging about just over 10 years ago, with info from the American Lung Association's website. 20,000 a year in 2003 or so was up from 2000 a year in 1980 or so, and the rise was in tandem with industrial air pollution. How many people died in the 9-11 attacks? Now we have a "war on terror;" where is the war on air-pollution for the far far greater number of innocent American children who die each year from asthma attacks, let alone those hospitalized?
Anyway, there's my beef for the day. Good thing I'm vegan. Would hate to eat that...since I drive a diesel now. With a bit more time (will the rain EVER stop long enough to continue my bus-to-motorhome buildout?), it will hopefully be hy-boosted, though, and that will eliminate 99.999% of the particulate emissions, with only carbon-monoxide as a hazard. Haven't had time to look into what it would take to do that with this diesel, though. Gas is much easier to hy-boost without backfire, so they say...
But add 5% of your fuel in hydrogen, and gain virtually all the available power from your base fuel; through away your catalytic converter and DEF system, and your exaust will still be cleaner- virtually 100% clean. All fuel will burn completely in the motor cylinders, without a higher temp burning out your valves (like nitrous can do), and your exhaust vapor is literally water. I've read they use these systems in giant enclosed warehouses that need diesel forklifts to handle the large loads. A DVD I got from a guy who "says" he was a professor at the University of Arizona (or similar) where they have studies alternative fuels since the 1960s, claims with hydrogen mixed in, ANY fuel (liquid or vaporous) can be used in a spark motor: diesel, low-grade alcohols, vapors collected from rotting debris, and more), and he had working demos, and showed what looked like a lab with impressive equipment in it, so I believe him.
I put a cheap-o creap-o home-made piece of junk hydrogen generator in my Nissan Pickup with a Weber Carb, and I could just begin to tell it had more power - but my H2 generator could not produce fast enough to support the needs of the engine size.