In this case I'd recommend the 2635 or 2636, they are going down to 1fA.

At an average of 1 electron every 150µs, the current flow becomes lumpy and digitised.
Use triax, and don't flex the cable while using it
If you are going to play "What is it" try this one. Its a part I have and yes that is an SMA connector on the end.
As we've ruled out pitot tube (and presumably anything close and pressure related) then my list would include liquid cooled load/terminator, a flow cell of some sort, or (low probability from the connections) some sort of GDT type lightning arrestor.
+1 on the liquid cooled load.
If you are going to play "What is it" try this one. Its a part I have and yes that is an SMA connector on the end.
Antenna with preamp?
It looks like it's got 2 distinct "ports" / tubes to let the air in. And it's Robert so unless he is pervert and is on purpose trying to mislead us, then it's probably an aircraft part. So.. some sensor in an aircraft that takes air in. But it's not an air speed / Pitot tube sensor. So.. maybe a sensor for an altimeter ??
Going along those lines, I'd choose a vario. But the "flask" looks a little small, they wouldn't
need an SMA, and I'm not sure it is sealed.
Part of the day quiz, Leather medal for grabs
I too thought at first that it was a VFD of some sort... and given the shape of the active elements, it looked to me like robot eyes, maybe for some stoopit robot toy from the 80s or 90s...

Thanks for the archaeology lesson!

mnem
Anybody can buy expensive chemicals to make the problem go away... 
Sn18Pb32Bi50 (Rose Metal) is pretty cheap and melt at around 95C
Yeah, but as soon as you shop it made into a convenient-to-use wire form instead of globules, it gets much more expensive. Could be fun to find ways to cast it into useful forms with boiling water tho.
mnem
*knocks self unconscious with a rosin mallet*
Globules might be OK for desoldering applications. If not, then your rosin mallet might help change the shape 
But would little Pb pancakes
really be any good for the world...?

mnem
great... now you've got me imagining a whole regiment of tiny little Gallaghers with tiny little Sledge-O-Matics marching across my workbench...
https://www.newark.com/metcal/mx-ds1/desolder-hand-piece/dp/41B5513
Oh... yeah. And tweezers too. And custom heads for DIP8/14/20, etc... and all the cleaners and... and... 
mnem

A kings ransom to pay for them too?
I find that winding the temperature up a bit and adding a load of fresh solder helps a lot when working with large ground planes.
This is mostly for those playing along at home:
Yeah, that seems to be a common misconception here and everywhere soldering is discussed. There is a huge difference between cranking up the temperature vs delivering more heat. There is a reason we have big wide chisel tips with lots of mass and contact surface and tiny conical tips to reach tiny spots and almost every variant in between, even with conventional soldering irons. While you
can make do by cranking up the heat on a smaller iron, it really is not the right way to do it, and often results in burning the substrate and lifting traces because right at the contact point you are delivering much too high a
temperature to the work.
The difference with MetCal/SmartHeat and similar HF/inductive heating technology... and it
really is something that has to be experienced to understand... is that it consistently delivers much more
quantity of heat in a smaller tip than any resistive element iron is physically capable of doing.
You really do get a quantum level higher
performance for your money.
The question then becomes one of budget vs need (or in my case, very limited space in my workbench bag) ;
there simply is no question that if you can spend it, MetCal is worth every penny.mnem
There is a Keithley 2602 SMU offered by zhoefler in the bay. Starting bid: 1190,- EUR (NAWTS)
https://www.ebay.de/itm/403162060138 
I give up. What is a system source device used for?
A digitally controlled power supply that can also measure it's voltage/current with 6 1/2 digits. And it's "4-quadrant" so +- on voltage and current - which also makes it an electronic load.
A true "swiss army knife". And two of them in one case! These range up to 20V/ 1.5A and 200V/0.1A. Also, they are priced like a small car...
These are the original non-letter units, long out of service. I have a similar "A" unit, but these are out of service as well. Only the newest "B" units still get service from keithley AFAIK.
Edit: I should add that they are superior to a normal DMM, as since they are power supplys you can set the current to zero - as long as you are inside the +-200V compliance range "no" current will flow in/out of the device - which makes them a lot higher impedance (infinite in theory) than the 10Meg of the usual DMM.
Yeah, Cerebus expounded in great detail on the joys of dev work with a Source Measure Unit when he bought his, but
eevBlog search is being a complete useless whiny bitch today.

It's like how we usually monitor a project with voltmeter/ammeter and a spaghetti of wires... only designed to do it all internally at very high precision
and provide CV/CC power supply
and generate graphs/logs of the testing which can be manipulated on your PC.
This level of precision even at low power usage makes it very valuable for characterizing the current/voltage demands of even single components within a design under changing operating conditions.
mnem
*currently having another pang of PSU envy*
If you are going to play "What is it" try this one. Its a part I have and yes that is an SMA connector on the end.
A pitot tube?
I was going to guess some form of BMAP sensor... but that's pretty close to a pitot tube, so prolly not.
mnem
You used to be able to get that stuff in every colour under the sun, probably still can if you search hard enough but it's become a little hard to find presumably
Hmm OK, will search hard then ! I fear I might have to get paint custom mixed though, in the end, as I find it highly unlikely that some thing off the shelve would match the Tektronix colour closely enough to blend perfectly with the remaining/good parts of the face plate. I fear this could become a bit of a rabbit hole for me...
Or you could just re-color
all the engraved markings so they match.

mnem