Here are a few pictures I have.
This is what you see when installed as a front wheel, note there is clearly no separate lock ring or rings.
http://rides.webshots.com/photo/2966649030080251109YGPdjz
It's easy to spot these Firestone RH-5° rims even at a distance.

Here is the inside, similar to the last picture Stu linked to above, the "lump" in the center inside is what you look for, a normal lock ring type rim is completely smooth here.
http://rides.webshots.com/photo/2962492320080251109cWSTan
Don't confuse this relatively low and flat "lump" for the more pronounced and angled drop center on a tubeless rim.

Grigg

•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-