Unless you're matching a current production example from a specific manufacturer, your comparison gives you no evidence that you received the incorrect item.
It does raise the question that 7075-T6 tubes from different sources appear to be exactly the same in color and wall thickness and the ones in question from a single source are completely different.
But you have a point so I have ask if anyone has a current production 6061-T6 tube which has the thinner walls like the 7075-T6 and light gray film coating? I wish I could post some pictures to show the gross difference in wall thickness but my digital camera is currently not available.
You are overthinking this subject. Buy from reputable dealers and you won't likely have this problem. The same goes for bolts. How do know if you are truly getting a Carpenter 158 HP/MPI tested bolt? You don't. That's why you buy from trusted sources or you ask said company and gage the response or lack of.
6061 is AFAIK used for commercial-style receiver extensions (cause if you wanna go cheap - let's go all out cheap!) Since the MilSpec version is more expensive to manufacture based on it's design from the get go, I would say the odds are pretty good that they are made of T6 as specified as well. Aside from that, what IG said.
Remove all doubt, buy from BCM.
As I have posted in the past, the TDP is specific on the details. They are 7075-T6, Hammer Extruded and dry film lube inside. To my knowledge, you are only going to get all these things from companies such as BCM, Colt, LMT, KAC, etc. There could be others, but these are the only ones I know for sure.
C4
I know Palmetto State Armory sells both 6061 and 7075 buffer tubes in mil-spec diameter. Right now, they have the 6061 tubes on sale for $13 which might be tempting to some people.
I talked to PSA a while ago and they told me all of their 7075 tubes have the grey dry film coating on the inside and their 6061 tubes do not.
Joe Mamma
I have the information I need as another friend has the same buffer tubes (Same Retailer) which are exact clones as the ones in question and his invoice was marked 6160. Since this just may be a simple mistake when they pulled the parts from stock I will contact them for replacement.
I can confirm that my recently purchased PSA 7075 buffer tubes do indeed have the grey dry film coating inside.