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I've got the Vang Comp on my 870. I like it with my TSA M4 adaptor. Haven't tried it on a field stock but I think I might pick one up for my brother's 11-87 for him to try during goose season.
I found that the Vang could be disengaged inadvertently, so I've stuck with the (Wilson Combat) Scattergun Technologies safety. And - don't believe everything you read - the Knoxx SpecOps works just fine with it.
Either Wolverine or The Shooting Edge may have them. They are also available from Brownells.