The smaller head size of the originals would indicate they were J.I.S. type fasteners. That's a major difference between them and regular metric fasteners, the smaller head sizes. The threads are the same, just the head sizes differ on some size bolts and on the Phillips screws. For bolts, the one we most commonly encounter on our 650 is the M8. Regular metric M8 bolts take a 13mm wrench size. The J.I.S. M8 uses a 12mm wrench. The six M8's used on the cylinder head are this type.
Sometimes the smaller head size is required. A good example of this is the handlebar switch assemblies. The J.I.S. Phillips screws used to clamp them on the bars fit into small recesses in the housings. Larger headed regular Phillips screws won't fit.