Have a 1973 F250 barn find.
the original voltage regulator is a small device about 1"X 3/4" x1.5"
with a rubber three conductor connector.

The ignition is breaker points and the engine runs all right.

Converting to the more common external flat style regulator with the flat four conductor connector.

The ignition coil has 7 volts on it in running condtion, since is downstream of the ballast resistor or resistance wire.

Looking for a good source of ignition 12 VDC under the hood, down stream of the ignition switch for the "S" terminal on the external VR?

 

Please fill in some missing information. Modern electronic regulators are plug-and-play to the existing harness (even as far back as '73), which means you're either looking at something else, or things have been modified. Can you provide more background? Please also describe if this vehicle has a factory ALT light or ammeter in the dash.

There is an ignition source under the hood specifically for the voltage regulator, but you'll need to fill in these missing pieces first.

 

It sounds kind of like you're describing the horn relay, not the voltage regulator.
The regulator is located on the passenger side of the radiator support, just below the horn.
The horn relay is on the passenger side splash pan near the battery and starter solenoid.
First picture is the horn relay, second is one of a mechanical regulator, solid state ones are a little "flatter".

 

Has a factory alternator light, no ammeter or volt meter

The first photo of the horm relay is what I thought to be the VR.
Missing the VR shown in the second photo or the solid state type.

Can find no wiring harness for the VR.

There should be a connector for the VR, either mechanical type or solid state type?

 

There should be a harness for the regulator unless someone has converted it to a one wire alternator but I doubt it (one wires have the v reg built in).
This is a not too clear picture of what you should have. It's a solid state reg in this picture.

 

Will look again.

Did not know where to look for VR, seems like an obsecure location compared to later models.

Maybe the the VR is so rusted like the one in the photo that it is camouflaged against the rusted fender.

 

This is the original setup. The alternator wiring harness is "Y" shaped, one branch goes to the alternator, another goes to the regulator and the last leg goes up to the battery terminal side of the stater solenoid.
The second picture, that's an "after" shot of the same truck that's in the previous post, the one with the rusty regulator.

 

The regulator may be on the passenger side fender, or tucked inside the radiator support like Mike showed.

What's ultimately your goal? Are you have a specific problem you're trying to solve? I'm assuming the charging system is included when you mention that the engine runs all right. Is it as simple as you suspect your truck has the original electromechanical regulator, and you would like the peace of mind in having an electronic version? Personally I would not be looking to fix something that isn't broken, especially regarding an item you haven't even seen yet.

Also note that the chances of a 40+ year old vehicle having the original regulator are slim. Not only do they naturally live out their life within that timeframe, they're also one of the most commonly replaced items (even when they don't need to be replaced) when a panicked previous owner has charging system problems, and throws money at the vehicle rather than troubleshooting. Voltage regulators, starter solenoids, and coils are prematurely replaced all too often because they're cheap and easy to access - which makes folks want to believe it's the problem, when in fact it usually isn't.

 

The OP has a '73 model so the voltage regulator should be on the radiator support. Later model years, I forget the year they changed it, had the regulator on the passenger side splash pan.
Like you said though, after 40+ years, there's no telling what's been done to these trucks.

 

Found the VR mounted on the passenger side radiator support with the wiring harness.
The VR was totally rusted on the top and side surface and blended with the rusted radiator support.
Suprised did not notice it when removing and restalling the radiator.

Believe the mechanical type VR is original equipment, since the botton side still had some blue paint over bare steel, ..the Ford procedure of painting.
If the VR was aftermarket, believe the cover would be plated.

Problem I have with the mechanical type VR is the cut out voltage setting. Some mechanical VR are adjustable, while others are not adjustable. This VR is not adjustable.

There is something called gassing voltage. It is the charging voltage where the water in the battery starts to evaporate. Basically, the battery is in a state of being over charged.
The gassing voltage is ambient temperature dependent. As the ambient temperature increases the gassing voltage decreases. The high end solid state VR have a temperature compensor that automatically adjusts the cutoff voltage, so the battery is not over charged.
This is similar to charging a battery with a single stage battery charger compared to a 2 or 3 stage battery charrger

Before the solid state type VR folks would turn on the heads lights when going on highway trips, so not to overcharge the battery, especially in the summer months with elevated ambient temperatures.