If no arrow, id always fit them with the solid inside face on the feed side, the solid machined side when you look inside the valve if that makes sense
Why, a fellow plumber disconnected the feed side of one of these in some flats, temporarily, as it wasn't letting by he for whatever reason didn't bother to cap it and went home, during the night the ball part blew out of the open end of the valve, the valve had no arrows, but if the valve had been oriented the other way it wouldn't have been able to happen