Well the dust on the heatsinks is probably just left over from manufacturing. Bitmain had to have the sinks made someplace then shipped to them to assemble.
I'm sure each heatsink wasnt cleaned prior to assy.
As for the design, maybe they had different batches made by different suppliers.
As long as the circuit boards are the same and say S3 on them then they are legit.
The heatsinks are not causing your problems.
You said you were going to let them run un-molested for a while.
Hows that going ?
This is my last offtopic post about this here, if anyone is interested in finding out more, lets take it to PMs.
My bad unit looked almost like this when I first opened it, I had less dust on it (like 70% of what you can see on following pic):https://bitcointalk.org/index.php?topic=330665.2280
The dust only was on air intake side of the unit, the other side was fresh clean. Moreover the intake fan wings were also covered in thick layer of dust, I hope I do not need to go further proving that unit was running for a very long time before I got it.
I asked bitmain about different heatsinks on same batch and dirty heatsinks - they asked me the order number and then held back to answer, but answered that there is nothing they can do unless the unit is faulty.
The lastest update from me is - good news:
1) on the pool I am on now - they do not penalize hoppers so I can restart the unit whenever I feel it starts to underperform
2) I ran the units without restart for 24 hours hoping to run them until end of the week, performance wise they ran as expected for 4-5 hours, then the bad unit took a plunge for 20 hours non stop underperforming and no sign of recovering (400 GH/s)
3) seeing that last night I decided not to wait until end of the week as I promised and took the good unit apart and by popular demand took pics to show different heatsink design on same batch units and to find that it is almost perfectly clean inside the good unit, just a little dust on it which most probably collected while running at my place, I cleaned that off, tightened heatsink screws, reversed both fans which caused the good unit run a lot cooler, too - fans ran 1900-2100 RPM before the procedure, now they run 1600-1700 RPMs, combined with the bad unit already cleaned and reversed fans now the units run at pleasant noise level compared to what they were running at upon receiving them. as the units generate less heat now, the room they are in is little cooler than it was. After procedure I started both units to find them running 880 AVG for 12 hours, then there was an internet connection problem for 1 minute at my place after which the bad unit started underperforming again, after restart the units now run good for 8 hours.
to conclude - the problem is still there, keeping room temperature low seems to fix it.