With the volumes they did I always wondered how they kept the film matched up with the owner; especially with sheet Kodachrome, but they always seemed to. I've never heard of anyone getting the wrong film back from a Kodak plant in Canada.
The films stayed in their envelopes all the way to the pre-splice room. As the Kodachrome was added to the large reel, each roll had attached to it one half of a numbered twin-tab, while the envelope received the other. The envelopes traveled with the films as they moved through the processing and mounting or being placed on the movie reel. Either a piece of the envelope with the full return address (in the case of the mailed films) or the envelope that the film came in (in the case of film from dealers) was used to return the film. In each case, the matching halves of the twin-tabs were together.
One of the things that my Dad's staff had to deal with was the small but not meaningless amount of film that came to them without a return address, or with an unreadable return address. Dad considered those stick on return address labels to be a scourge, because of the number of times that they would get separated from the film envelopes before the film ever got to the lab.
Of course, in addition to the un-addressed films, and in some cases films that didn't have the owner's names on them, Dad's staff also got to deal with the letters and calls from people whose films appeared to have gone missing.
Dad discovered a company selling an interesting, highly analogue "computer" response to the problem. It was a mechanical "matrix" that resulted in cards with holes punched in them. Dad's staff would laboriously go through unlabeled films and try to categorize their contents. Those categories would result in holes being punched in cards at appropriate locations. There would be a particular spot on the matrix for something like a Christmas tree. If there was a Christmas tree on the roll, there would be a hole punched on that space of the matrix.
Then Dad's staff would communicate with the people whose films were missing and learn what was on the films. A matrix card would be punched appropriately, and Dad's staff would start comparing cards to see which two cards had holes that lined up. Sometimes it worked really well, and people got their film. In other cases, especially when people's memory of what was on the film was poor, it didn't work as well. And of course, categorization was at best inexact. But a fair number of those orphan films reached their owners.
As for sheet Kodachrome, I don't know - the North Vancouver lab didn't have machinery to process it. I don't know that the volumes were ever that high for it. I would assume that they would have used the same systems as any commercial lab that developed sheets of any kind for customers.