Just a guess, it may be the lense... yours is a 6.8mm and most tend to be 13 or 19mm.
Mine is Ver1 recently upgraded last year to Version 2: 640 Full Radiometric 19mm and it does have the round ft... but I was assuming either lens or lens thermal filter. There doesn't appear to be any tang slots to "unscrew", although there are internal threads probably to initially calibrate the lens.
If it works as expected, I'd assume the ft is not a heat range filter (as other brands can change if desired).
That means it's either a lens ft glass element or some type of protective clear cover.
If you don't get a explanation, I'd just send DJI service a quick support question... you can start process to create a ticket, during that process, it prompts to email or chat to resolve quickly... you could then ask directly.
Curious as to your findings?