I have previously tested Made in USA classic Stanleys against Made in China classic Stanleys (   • U.S.-Made Stanley Thermoses vs. China...  ). You can see the results in that video or in its description if you don't want to sit through the whole thing.

In this video I took the best-performing Stanleys and pitted them against Stanley's new (Made in China) Master Series which is supposed to keep liquid hot up to 40 hours and have new "quad-vac" technology instead of just the traditional double wall vacuum insulation.

Here are the results of four flasks of 203-deg. F. water left in mostly single-digit temps for 30 hours:

1980 (made in US and best performer of previous test): 122.4 deg.
1998 (made in US and not previously tested): 74.5 deg.
2016 (Made in China): 112.7 deg.
2018 Master Series (Made in China): 125.4 deg.