This is a most interesting, and intriguing, subject to me. I do not want to bore everyone with old stories, but I thought a little background information might be appropriate.
I lived in Japan (approx 25 miles north of Tokyo) for four years (1960-1963). I spent most of my leisure time visiting Japanese sword shops and various Museums in Tokyo in order to learn as much as I could about Nihontô. It was quite a struggle, especially since I spoke very little Japanese in 1960/1961. The only Nihontô books written in English that were available to me at that time were "The Samurai Sword - A Handbook" by John M. Yumoto and "NIPPON-TÔ: THE Japanese SWORD" by Inami Hakusui. And, Of course there was no Internet available for research in those days.
The monetary exchange rate then was 360 yen to the dollar and Nihontô blades and koshirae were very inexpensive -- you could buy a nice kotô wakizashi in full polish (in a shira-saya) by a good smith for under $100 -- and services such as polishing or having a tsunagi made were likewise very cheap.
But I digress. Sometime around 1961 (I am not sure of the exact time etc. for it was so long ago and my memory is now not good) I bought a nice katana at the Japan Sword Shop in Tokyo -- the blade looked healthy to me with no problems (in my neophyte eyes), but I thought the koshirae could be improved on (ah, the folly of inexperience!). So in a flash of inspiration (?) I decided to have the blade repolished, a new tsuka and tsunagi made and nice "en-suite" tosogu installed. Inasmuch as I didn't really have a clue what that meant I asked Inami san (sensei) to select appropriately matched tsuba, fuchi kashira and menuki. I am sure he muttered some choice Japanese utterances under his breath, but the koshirae looked beautiful to me when I got the sword back. I still do not know what criteria he used in selecting those fittings -- or if indeed they were properly matched -- I still do not have a clue as to what correct matching means anyway.
BTW. I long since sold that katana.
Please visit this page to see what I believe is a wakizashi Satsuma blade in koshirae made up for the Tourist Trade. I wonder how well the tosogu are appropriately matched?