metallic debris in oil filter

During 700 hrs oil change i'd found some debris in oil filter paper.  < 10 small pieces - non magnetic  (photo in attachments). less than 1 mm.

at 550 hrs sprag clutch was changed (rear carter opened) but in the following 100 hrs oil change (at 600 hrs) oil filter was almost clean of debris.

other data : magnetic plug completely free of debris. compression and leak down tests are perfect. Boroscope analysys ok.

I'd had the suspect (with no data evidence) that the engine had an overspeed for propeller governor malfunction and it seems that there is a subtle power loss 

(bacause i remember an old post i'd like to ask ) can these metal pieces come from lifter surfaces or are this type of debris magnetic? 

or is ths level of debris normal 150 hrs after sprag clutch service with carter opening and working inside the engine?