Yes Argent was the correct color for some of the Ford silvers, but the early Koenig products was brighter...more silver than Ford's Argent.
I remember it being more dull than bright but it's been a "while" since I've seen a factory fresh Koenig ( : < ) and you could be correct. Again, if the original shade is important to the new Koenig owner, a quick call to Koenig could clear it up. But I think he'll do what most of us would, paint it what color he likes!
On that subject: I have a 1970 Koenig catalog and while there is NOT a separate features & specs page for the Bronco, as there is for all the other 4x4s of the era, it's in the price list with part number 536B for the six and 536C for the V8. List price is $420 for both (about $2600 bucks today). The bumper looks very much like the one used on the 800 series Scout and the mounting of the Scout 4 cylinder version is similar to the Bronco six, as well as the V8 setups for both being similar... yet more Scout/Bronco connections!
Koenig must really have had a locked in agreement with Ford because the ads I see from Koenig in the '60s always list the Ford part number and tout the "approved by Ford" aspects.