Have you got the lens, or just shopping for it now?
I ask because the tripod ring is included with the 100-400L. So, unless you have the lens already and it's missing the ring, you shouldn't need it.
If you do happen to need it, it's the Canon Tripod Ring B (w). The (w) stands for "white" to match the lens. There is also a (b) or "black" version.... same in all respects except for the color.
The Tripod Ring B (w) fits the 100-400L, 300/4L IS, 300/4L, 70-200/2.8L, 70-200/2.8L IS, 70-200/2.8L IS Mark II, and 400/5.6L. (and maybe some others).
The black version Ring B that's used on MP-E 65mm macro, 100/2.8 USM macro (not the 100L macro), 180/3.5L macro, 200/2.8 and 200/2.8 II (maybe others), also would fit and work, just won't look as nice.
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
FLICKR & PRINTROOM
