[EsBee]

I just purchased from someone a month-old copy which was a replacement by Pentax for a dud. I haven't run a whole lot of tests but so far the images have been great!

[and]

Im actually more happy and impressed with the 16-50 than the 50-135 so I am quite surprised by that. my copy is great

[Burgerbrater]

The serial numbers can tell you approximately at what stage the lenses were manufactured - the serial number database goes into this more. Then again I doubt that Adorama would have sent you 3 first generation 16-50s and even then odds were against you having decentering on all 3! You were just unfortunate.

You could, of course, try another vendor like B&H, who have a similar returns policy but may have a newer (and non-decentered!) copy to ship you...

Ash, I went through the entire database. Read every posting. However I cant really see how one can determine the manufacturing date by looking at the serial number???

[Ash]

There's no way to determine accurately when each lens was manufactured.
What's more important is if the lens bought new now came from the initial batch (higher decentering rate) or the second wave of production (fewer overall issues). Serial numbers starting with 901 were more likely to be from the initial batch, but as seen in the database, there are a number of reports of failures regardless of the serial number - again I consider this responder bias, and can't be in any way an authoritative measure of failure rates.

In any case, there seems to be more user confidence in the second batch, which in many threads has been discussed as more reliable in IQ - SDM failures are, of course, another story...