A final thought, because I think it's easy to get so caught up in the trees that we lose sight of the forest:
1. When Tomlinson and Wright were shown the bullet in June of 1964, it had been in the FBI's possession for nearly SEVEN MONTHS. All the testing was complete. The FBI knew it was from Oswald's rifle.
2. Oswald had been dead seven months. There was never going to be a criminal trial. Chain-of-custody technicalities were irrelevant except to the extent the WC wanted to be satisfied.
3. The WC burdened the FBI with many requests, including a half-assed chain-of-custody request that many items of evidence at least be identified by the first person in the chain. For many items, this was done with a mere photograph.
4. When Odum showed the bullet to Tomlinson and Wright, he was merely on a routine mission to satisfy the WC. This was no big deal. The FBI had even abandoned the requirement for 302s because the requests were so burdensome.
5. The responses of Tomlinson and Wright would not have caused Odum or anyone else to think "Oh, my God, WE HAVE A PROBLEM WITH THE CHAIN OF CUSTODY!" They would simply have thought, "Well, we'll have to report what they said and go up the chain until someone provides a positive identification." If the FBI had thought there was a major problem, the documents would have simply said "Due to the passage of time, Tomlinson and Wright were unable to provide a positive identification but said the bullet looked like the one they found. Let us know if you need more."
That would have been the FBI's perspective. Yes, it's not the perspective of CTers who think the WC should have been a criminal trial of Oswald or that the FBI should still have been preparing for a criminal trial in June of 1964.