Metal flakes may be the residual evidence of prior treatment with a radiator stop leak product.

Unfortunately, the heater core may still require replacement at some point.
For your sake I hope not.
It's doable as a DIY, but very tedious work and expensive if you hire a shop to do the work.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022