I'm trying to tell you there is not "an extension part" you seem to have "a hub" and a drum. The hub is one piece, only one part, does not disassemble.

Take the spindle nut off and see if hub and drum will remove.
Back off the brake adjusters if you can. Perhaps unbolt wheel cylinder and hope wiggling it around to create slack in the brakes if no luck with the adjusters.

You might be able to unbolt hub from drum or may not without access to the inside, and even if you can remove the nuts may need to press studs/bolts out to free the drum from hub. This all goes easier after hub and drum are removed from spindle if at all possible.

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-