I still haven't found a definitive guide to detecting the difference between #357 and #500 US Army Land Rovers. There are slight colour shade differences but that is about all I can see and, for all I know, those variations apply to each.
The best summary I can make of all this is to conclude that there is no difference. They are the same. Corgi just decided, for reasons best known to themselves, to make the export model No. 500 and hurriedly made a temporary box for it and never got round to making a 'normal' box for #500. That may well have been because there were no plans to have a US Army Land Rover in the main collection in 1963/4.
No.500 seems to have been first produced in November 1963 but is said to have been withdrawn in 1963 too, making for a damn short production run. These were exports for the US market and, if the dates are accurate, that was the only example at the time so there wasn't a model for British kids.
Model No. 357 is called a Land Rover Weapons Carrier and appeared quite a bit later in January 1965 together with a whole pile of US military models. Until then there had only been a couple of RAF items. No-one here would probably have known #500 existed.
This is a genuine #500 box that an American boy will have found his Land Rover in had he been lucky at Christmas 1963. That was the last chance he had to get one and the books tell us that the sales were around 17500. That is one of the smallest figures across the whole range.
This is what British boys will have got in 1965, and maybe had to wait until Christmas that year despite its official launch month being January, most families recovering from over-spending the month before anyway. It looks pretty much the same to me.
The new box, and this is another genuine article, has a nice picture and no 'US Army' text but the smart Land Rover logo and 'Weapons Carrier'. The star does rather give things away a bit but it sold a few more until itself being withdrawn in 1966. I don't know when but that means rather less than two years, over which a creditable 130000 are said to have been made.
Of those, some may well have been fitted with red interiors, by production workers who had either run out of yellow and nicked some from the brown Land Rover bits boxes or, maybe, just got bored with yellow. They are certainly rare. #357s with red interiors are very rare so a #500 with red interior would be extraordinarily extraordinary.
So perhaps I will say that the one illustrated at the beginning is a #500! Unless, of course, you know different...