If the assumed to needle bearing pieces came from one or more of the current lifters, it will be obvious to you if you measure the rocker arm lash on each lifter and discover one that is excessive. If they are all in spec, it was probably from the previous lifter that was replaced. You can buy those big round flat magnets at Harbor Freight and stick them to the bottom of the oil pan to capture and discourage future ferrous pieces from finding their way into your oil pump.
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