Hi K.O.!I can't really see them that well on my phone but Slingerland did use sort of an oddball bolt for their lugs for a number of years. It's a flat head with a cross design but it's like two regular slots crossed, not like a Phillips head. I've never seen this type of head anywhere else.
What you have are not those but may be correct for earlier drums.
Those are fillister head bolts.