I'm not entirely sure, but I suspect a gasoline issue needs to be addressed.

History:

The gas tank had old fuel sitting inside. I drained that fuel ( it was a beat red ), and put tank cleaner into the gas tank tank.

All that was drained, and I ran clean fuel through the system with the hose after the fuel filter disconnected from the carburator intake, and this newly added fuel was deposited into a gas can at the rear are of the trunk, and it was rinsed BEFORE going any farther into the fuel lines out to the engine compartment. This process was done over ten times with new fuel. The system ran clear and clean.

I then replaced the fuel system with new fuel filters. One fuel filter was put in line immediately AFTER the fuel pump located in the rear trunk ( Bonnet is the English term, I'm told ). Another fuel filter was installed inside the engine compartment immediately after the metal fuel line from beneath the car. That fuel filter was then connected to the glass bowl fuel filter that ultimately was connected to the carburetor. So there were three fuel filters AFTER the tank cleaning/Fuel line rinse.

This seemed to work fine. The fuel pump did pump out clear, clean gas. There was a leak in the SUD carburetor that needed to be addressed.
( The rubber diaphragm inside the SUD was old, dry, and cracked. I'll post pic later ). So there was a small bit of lag time.

With that SUD repaired and the the fuel lines tightened with quality hose clamps, there weren't any more leaks. The fuel pump worked intermittently, for some unknown reason. But when the fuel came out again, it was an amber color ( see pics).

This fuel wasn't as dark nor as red as the first rinse, but it was out of the ordinary. I disconnected the class bowl filter ( it began to leak ) and ran the fuel directly into the new fuel filter. Little bits of debris can be seen.

The flush/rinse process was repeated again, and now the fuel seem clean and clear, as it should be.

Any thoughts on what caused this or what might be done so that this amber-brackish fuel does not return? Is this significant to the engine and combustion? Any thoughts?

 

After reading you post a few times...
I would have to say that some old diluted fuel (diluted from you gas tank cleaner)
was missed and it was simply pumped up into the system.

If you were to remove the gas tank sending unit located inside the boot through the access port, and shine a light in there, I'm betting you will find a layer of rust in the tank.
That would account for the bits of debris and your filter.

 

OLD CAR,OLD MEN AND OLD GAS....Im posting a new thread in 5 minutes.....
Its probably a mixture of gas stabilizer with the old gas....as I keep repeating , the gas is crap...once your that far gone its all got to go!
Read the new post Im starting....

 

I have not split open a MK2 gas tank, but I understand there are a number of baffles in there. Even if you pulled the tank, it is difficult to get it completely clean. I expect that is where you are getting the old gas and crud in your system.
However, the filters protect your carbs, and you have probably done enough. Just use up the old fuel by driving the car, and over time, it will be pretty clean. The best thing you can do with an old car is to drive it! Understand this is hard to do sometimes for a variety of reasons.