How do you know it's OEM? 2001 330CI Coolant Expansion Tank

Thanks for excellent posts on replacing components of the cooling system. They were great in process and parts list. After reading them, I ordered the parts from prominent suppliers specifying some parts to be OEM. See parts list below.

Today I received the Coolant Expansion Tank. How can I tell if its OEM? How can I have confidence in this tank?


Observation:

1. There are no manufacturer's marking on the tank. The only markings are "Germany" and "PA 66 GF 30" in raised letters!
2. There is a coarse, very rough, circular section on one surface about 1 inch in diameter. Other surfaces have shiny smooth texture. The contrast in sheen and texture suggest post production modification. I wonder if this area is thinner and weaker than rest of the tank!
3. One surface has glue residue - possibly from a label that was removed!!!

Vendor's web site indicates they aren't permitted to advertise the OEM actual brand and assures their personal verification ... I understand not advertising the manufacturer brand, but should it not appear on the product? I've contacted the vendor with the question also.


Parts list (bimmerspecialist.com)
17 11 7 573 781 $82.99 Coolant Expansion Tank (O.E.M.)
17 13 7 553 919 $10.99 Fluid Level Sensor (URO PARTS)

Parts list (autohausaz.com)
17 11 1 437 362 $48.55 Thermostat for Expansion Tank (OES)
11 53 1 436 408 $27.24 Radiator Hose; Lower to Thermostat Housing with Temperature Sensor Fitting (OEM. CRP Contitech)
17 12 7 510 952 $31.02 Radiator Hose; Upper Radiator to Thermostat Housing (OEM. CRP Contitech)
13 62 1 433 077 $16.14 Radiator/Auxiliary Fan Switch In Lower Radiator Hose (FAE)