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To have a really good shot with Agaricus we need more info.
For a start, take a fresh specimen, cut it in half vertically, scrape the inside hard enough to bruise the flesh with your knife edge, and watch for bruising reactions.
Second, do they have a distinctive odor? The odor in these is usually strongest at the base of the stem or near the gills. If there is no obvious odor, try crushing the stem base with your fingertips and sniffing a bit more.
Third, a good detailed view of a very young ring is very helpful.