Having received a replacement rubber weather seal in the post this morning, I thought seeing as I had no football today get straight on with replacing the very worn one on my Canon 2x Extender. I have no idea how it became so worn as it is seldom used, probably less than a dozen times in the last 5 years.
Anyway a simple enough job, undo screws on the lens mount, lift the mount & slide the seal over, re-seat the mount and do up the screws, all in about 5 minutes, then a couple of test pictures to ensure that everything is in alignment and job done.
The Part Number for the seal is YA2-3463-00 and I believe the item is the same for most if not all Canon EF lenses that are weather sealed. I have a couple of other lenses that could do with the seal being replaced so I will confirm in due course.