I have seen these for sale before on you know where...
Very difficult to tell, experts (and I'm not one) are hard pressed to distinguish. IMO not many so called NOS 39s exist some would lean toward aftermarket. Good luck in your quest.
Distinguishing between Genuine and Aftermarket T39s
May 15 2012, 4:46 PM
In my experience, the only way to determine a genuine T39 from aftermarket is:
Installation of a genuine T39 on a 1665 will be a perfect fit - meaning that the watchmaker do not have tweak or adjust with much difficulties.
For a genuine T39, the clarity of the crystal against the dial is a critical factor. Aftermarket T39s placed on a 1665 will have a blurry effect compared to the real deal. Lume plots, dial details are generally blurry.
I have had supposed genuine T39 crystals installed and viola, the watchmaker has to force and make adjustments until the crystal is in place >>> This is a dead-giveaway that it is not genuine / factory T39s.
Additionally, aftermarket T39s are more prone to scratches than the genuine ones. Tried and tested.
Unfortunately, I had to learn it the hard way, so be careful on where you buy your "so-called" genuine T39 Superdomes from.
Like many have said, there can't be that many leftovers in the bakery which keeps surfacing on the market every now and then.
The above statements are based on my own experiences, and I would love to hear more from the seasoned collectors on my findings.