Opened up an old 400 - what is this???

I popped the heads off an old 400 I got from a buddy of mine. This is just a stocker I plan to use to get my 66 Dart back on the road while I finish the hotrod motor (470).

Under the passenger side head, there were some metal chips in one of the small water passages (I believe it's a cooling passage). They look more like drill chips than pieces of thread. They're also really clean and kinda fresh looking. Any idea what they could be from, or why they'd end up where they did??? They are marked with blue arrow. Could they be remnants from the last round of machine work?

I also noticed whoever rebuilt it (has 30 over pistons), stamped all of the cylinder numbers on the deck surface (yellow arrows). Why would someone have done this????

Overall, I'm very pleased at the condition of it. It should work well to get this beater back on the road.

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Here is a close up of the chips.

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