And these are similar to my original 4! So I took a closer look at my 150ml bottles and found more differences! I think those I got from Amazon are fake!
The first problem is the fakes are noticeably softer/thinner. They're also not uniform. There are some spots on each bottle where they dent easily if you press them. As I mentioned, one had a permanent dent after steaming. The originals are hard all around. So are the 8oz bottles.
The second, most objective, difference, is the weight/mass:
Original on left, fake on rightThe original weighs 21.5g, the fake weighs 19.9g. (note: measure more bottles to improve confidence)
Third difference, and easiest to check. The bottom:
Original says something like -25°C - +110°C. Fake has no wordings, but has these 2 arc-shaped impressions. I think these are used to eject the bottle from the mould. 8oz bottles are like the original, words and no arcs.
Last difference, the most interesting - colour-shifting ink! Not sure if this is an intentional copy-preventing feature. This is harder to capture.
Same original bottle, different angles
Same fake bottle, different angles
Ok the original bottle, when held at a reflective angle, has ink that appears silver, but turns darker when viewed straight-on. You'll see that the fake's ink appears the same colour regardless of angle. Again, the 8oz bottle also has colour-shifting ink.
So, I have good reason to believe there's something up with the bottles, and they're probably fake. I just ordered 2 more sets of 5oz and 8oz bottles, both delivered within US, so we'll find out more.
Bonus: this might be an issue with the fake caps, but my instinct says it's just bad design. The cap has an inner rim that flattens easily. In the picture below, from the 12 to 3 o'clock position, that's the original rim. From 7-11, it's flattened outwards. Once this happens, the cap will leak.