I've had many positive experiences buying from cousins and all the parts I've received have been legit, but I will note one thing. When I first started buying from Cousins I recall they used to have "Generic Omega crystals" advertised as "with and without logo." I don't see them on the site anymore, but It struck me as odd at the time. They had a similar crown section. They didn't have a ton of crystal/crown inventory when I placed my order, so I never bought any to test out. I know they have a ton of NOS Omega inventory, but these sub 10 GBP omega crystals didn't seem to be that considering I purchased some sealed crystals from them for 40+ GBP. I'm not calling out Cousins or making any accusations--just sharing a discrete observation.
Assuming these are improperly branded generics, such parts weren't outside the norm in the US from otherwise legit sellers in the last decade or so. For a while, some watch parts sites would sell "generic" Rolex crowns that conveniently had Rolex logos on them. Seems like Rolex quickly caught on and put an end to the practice.
I don't mean to unnecessary extend this debate about fake vs defective parts, but I've come across plenty of apparently vintage parts that appear to be obvious production errors, so I am now hesitant to immedately rule anything out. I make no comment about the preceeding conversation, but I just share this tidbit. I have a few friends that travel around the US buying up watchmaker estates and old stock houses. I sometimes get asked to help them identify unlabeled stuff. Just last year, one of these friends bought a large inventory of Omega parts from a deceased watchmaker's family. I think the watchmaker had retired in the 90s after having his own shop for a few decades, so all the parts were old. Many came with original order slips attached or envelopes marked "Norman Morris Corporation" so I didn't have reason to doubt their authenticity (I'll leave it an open question). I found both NOS mismarked endlinks (Ie. stamped 570 instead of 511, or some endlinks stamped and some not at all), and some seemingly defective crystals (ie. light logos, optical aberrations, sizing discrepancies). This may cetainly be an anamoly but it got me thinking about QC back in the day. Admittedly, I never considered looking for misaligned logos.