All CB750 threads are ISO metric. Honda switched from JIS during - some say at the start of - the 1967 models. 6mm and 8mm screws did not change for "standard" pitches, the fine pitch (not used on our bikes) did, I think. Smaller fasteners did change and it's a bugger when working with bike models that spanned the 67 change and you don't know that a part was replaced with a newer/older one.
Chrome is not going to tap easily.
Try to measure the thread pitch. It isn't easy with that small a hole but I've stuck paper in a hole and run something down against it, then measured the marks left on the paper.
These ones are 8mm on all bikes I remember. I doubt the K0 is different?
8mm can be 0.75mm, 1.0mm, or 1.25mm pitch. ISO Honda bike ones have all been 1.0mm - at least I think so.