The chambers on the .455 TL were bored deep enough to take either the .455 MkII or the .455 MkI cartridge. The MkI (aka .455 Colt in Canada) was a longer round that was originally loaded with black powder. It was found when they switched to Cordite that the shorter MkII cartridge was more efficient.
Are you sure that the chamber ridges you are seeing aren't just the normal restriction?
You would need to compare it to a known unaltered TL to be sure.

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Dean
SWCA #680 SWHF #446