The following information came from a very reliable source. I’m confident that it is accurate.
The first .280 loading by Frankford Arsenal was during January, 1950 when the Arsenal loaded a small quantity of .280 cartridges, with FA bullets, in FAT1 cases headstamped FA * 48. FA was very likely aware of the UK .280 developments and decided to obtain some preliminary ballistic data. Test firings of this round at FA were in a Mann Test barrel.
The next loading was in early 1951 when FA loaded a series of test rounds using UK bullets in FAT1E1 cases headstamped FA 48 and FA 50. (The cartridge that Fede attributed to Paul Smith may be one of these, although I’m still waiting to hear from Paul with details of his.) Reportedly, Springfield Armory also hand loaded FAT1E1 cases using components provided by FA for tests in one or more “T” series rifles which had been chambered for this cartridge. As far as I know, all FA tests were conducted in test barrels rather than rifles.
(As a side note, FAT1E1 cases, headstamped FA 50, were also loaded with T104 Ball bullets and tested by SA in one or more “T” rifles. This is a very uncommon cartridge, not usually found in collections. I have only seen one, which I own. I do not know who loaded it.)
Finally, in late 1951 - early 1952 FA loaded test quantities using the FAT1E3 case headstamped FA 51 and FA 52 using bullets provided by the UK. Firings of this loading at FA and Edgewood Arsenal were in test barrels. These are most likely the cartridges that have been shown on this thread. It is also possible (and I think very likely) that SA chambered a LR for this round, but this has not been confirmed.
So it would appear that FA and SA were both testing .280 cartridges as early as were the British and Belgians. I think there is a common misperception that the US dismissed the .280 as a possible NATO cartridge, out of hand, when the opposite is true. They did consider it enough to do actual testing of different cartridges over a 3 or 4 year period but, as we know, opted for the .30 caliber, for various military, and political, reasons. But, that’s my own opinion only so take it for what it is worth.