Fine bronze dust in diff oil

[Graham O]

I have now done 70 fun filled miles in the car and there is a noticeable whine from the diff on the over run.  The whole rear axle is a later one from a 36 Ruby which was already on the car.  Obviously I put new oil in during the re-build (SAE140) and so I decided to check the condition of this oil to see if it gave me a clue as to what was wrong.  The oil came out somewhat dark and I soon collected a number of small metal flakes with a magnet.  Thinking that maybe they were a result of my poor cleaning of what was a very sick diff casing, I was not too worried, but then I looked at the oil in the sun.  It is completely full of extremely fine bronze or brass dust which gave the dark appearance out of strong light. They are so fine that they don't seem to want to settle.  Where is this coming from?  The only part that I can think of is the oil thrower.

Now to admit some naughty pre-conditions.  My car was sold with a rear axle problem. I found what looked like aluminum paint instead of oil inside and all the pinion bearings had collapsed and bits had come into the diff.  See photo It must have sounded terrible.  So I managed to find a matching number pinion and crown wheel which are installed - aligned as best as I could at the time.  (I aligned it using red paint, but now that I have gear blue a second try is in order).  
The torque tube would not come apart as the oil thrower would not come out with despite lots of heat and a C spanner almost cutting off the lugs. As the pinion seemed to spin cleanly I decided to run with the replacement torque tube/pinion assembly as is.

Purists will be cringing, and no doubt I got what I deserved; but even so, were has the bronze dust come from? Remember, none of the parts in the photo have been re-used.
Graham