Hi all

I believe the key is in dbcc page with the:

Allocation Status

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GAM (1:2) = ALLOCATED

GAM pages record what extents have been allocated. Each GAM covers 64,000 extents, or nearly 4 GB of data. The GAM has one bit for each extent in the interval it covers. If the bit is 1, the extent is free; if the bit is 0, the extent is allocated.

The key is clearly identifying the GAM page.