Hello Flying10UK
This must have been a long skirted barrel that had been cut down to a short skirt. Because a long skirt is about/or is half way around the diameter. Cut back was common due to damage during the handling or installation process. After testing it was found that by cutting/machining off the damaged area that it didn't affect functioning. This helped salvage these barrels. I believe it was around mid production that a change was made calling for all new made barrels to have the short skirt.
That's why we see early made barrels with the trimmed, short skirts.
Though the one in the auction doesn't appear to be the prettiest job done. But would be fully functional as far as I know.
HTH,
Charlie-painter777