Can anyone explain how these lens ID codes work? Why must Tamron choose an ID code from an obsolete EF lens? There's nothing that I can see in the EXIF information that would indicate the lens code, so I'm guessing that this is unrelated.
Here is a simplified version as I understand it.
1. Each different Canon lens has a ID code which is sent to the camera body when a image is captured.
2. The firmware in the camera body has a lookup table of lens codes versus description.
4. The camera inserts the lens description into the EXIF.
5. If you have a Canon extender, it modifies the code so that the extender information is included in the exif as well. (If you have two extenders, only the one nearest the camera is recorded)
6. Many photo editors allow viewing of EXIF information, and may have their own way of lookiing at exif data and combining it to give better information.
So, it is possible that some lenses ID codes are left out, or new lens models and codes do not yet appear in camera firmware. They probably will never update firmware in out of production camera models.