I have looked at the paint work on the saya very, very closely, and think it is the original paint, so you think a IJNF sword? maybe? or it is an IJN special order? I thought I have also seen black saya's in pics before. The heart w/ mon is probably as you both have introduced as possible. No way to really know 100% now. I like it, it is a nice personal touch. The soldier must have been an interesting fellow.