I gave you a possible explanation. They wanted to see if others in the chain of custody could identify CE399. But, as I explained, it isn't necessary. Since they didn't mark the bullet they would have to rely on their memory. Memories of such details fade with time. However, none of them said that the bullet CE399 wasn't the one that they had received and passed on - just that they could not be sure.
So the WC just did that for the fun of it? Are you really that naive? And, none of them said that CE399 was the bullet they passed on either. I'm not sure where you get the "just that they could not be sure" part from, because that's not in SAC Shanklin's Airtel.
The purpose of a chain of custody is to confirm that the bullet CE399 was the one that was found by Tomlinson. The evidence of the 5 people in the chain established that to the satisfaction of the WC because the alternative was to speculate that one of them was lying and had substituted a bullet that had actually been fired by Oswald's MC. Such a conclusion would mean that one of the five was part of a plot to deceive the WC. The WC did not think that there was any reason to draw such a conclusion. Perhaps they felt, as Prof. Feynman explained, that one applies common sense and experience to the argument "well is it impossible that x could be true?".
The purpose of a chain of custody is to confirm that the bullet CE399 was the one that was found by Tomlinson. Indeed... Confirm, not assume, as you are willing to do!
The main purpose of a chain of custody is to ensure that the evidence relied upon in court actually relates to the crime and isn't tampered with or otherwise contaminated.
Such a conclusion would mean that one of the five was part of a plot to deceive the WC. Exactly, and that's precisely the possibility that the WC wanted to eliminate with their request. There is no other reason for it.
The WC did not think that there was any reason to draw such a conclusion. Of course they did. That's exactly why the WC asked the FBI to go back and check if the first four men could in fact identify CE399 as the bullet they had seen.
The WC did not know that. In fact, according to the FBI memo (CE2011) even OP Wright did not know that in June of 1964 - he just could not confirm that CE399 was the same bullet that he saw on the stretcher. Unless he took a photo or wrote a detailed description of the bullet, how does Wright "know" in 1966 that it was not the same bullet if in 1964 he was just not sure? How does his memory get better over the next two years?
True, the WC did not know that in 1964, but we are discussing it now and we know it. As for the FBI memo (CE2011), there have been - as far as I know - four requests to the National Archives for the FD 302 reports by Odum and Todd about the showing of the bullet for identification to Tomlinson and Wright and Johnson and Rowley respectively. None of those searches yielded results. As far as the National Archives are concerned, those reports simply don't exist. All they have are two airtels on the subject; one from SAC Dallas dated 06/20/64 (dealing with Tomlinson and Wright) and one from WFO (FBI HQ) dated 06/24/64 (dealing with Todd, Rowley and Johnson).
Add to that that Tomlinson is on record as confirming twice that only SAC Sharklin showed him and Wright a bullet for identification once and that was about a week after the murder and that Odum denied, in 2002, having ever handled or shown CE399 to anyone and that there is even no evidence that CE399 was actually in Dallas when Odum was supposed to have shown it to Tomlinson and Wright and you end up with the extremely questionable memo that is in CE2011.
Btw, three days before sending his now famous Airtel, SAC Dallas responded to a status request from HQ about this matter with an Airtel in which he said that they were encountering some difficulty. The obvious question would of course be what kind of difficulty could they possibly encounter when all they had to do was to show a bullet to two employees of Parkland Hospital? But there is a second question to be asked also; according to CE2011 SA Odum showed both men the bullet on 06/12/64, so what kind of difficulty could SAC Dallas still have had 5 days later? Perhaps the answer is simply that Shanklin never directed Odum to talk to both men because he understood that CE399 wasn't the bullet he had shown to Tomlinson and Wright in late november 1963.
Unless he took a photo or wrote a detailed description of the bullet, how does Wright "know" in 1966 that it was not the same bullet if in 1964 he was just not sure? How does his memory get better over the next two years? If Odum never showed CE399 to Tomlinson and Wright (which I believe) then Wright never said that he wasn't sure and his memory didn't need to improve as he only made one statement about that bullet in 1966.