correction it is the earliest shares.
That would be 5/23
when you sold 488 shares, those were 5/23 shares.
when you sell you sell FIFO,
if there's any 5/23 shares left you adjust their basis to the extent possible.
if there's any loss unallocated then you adjust the 8/21 shares
until the loss is entirely allocated.
if you had a profit on the 5/23 shares then none of this matters.
