Their is a notch or two in one half of the rim that will allow the other half to cock and slip past.
This is what they look like inside, notch not shown.
https://picasaweb.google.com/lh/photo/96LV_PnM-bC97_4AoWK_G9MTjNZETYmyPJy0liipFm0?feat=directlink

Someone has posted a link to a service manual showing the disassembly method, I can't seem to find it at the moment.

•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-