Hi
Glad you got your sword,a good start.
The paint on the nakago,is probably assembly marks,encountered in blue,yellow,red, it helped parts assembly of tsuba, and tsuka,handle etc,at the selected outfitters. I believe this is their purpose.
As for parts you could have a leather saya made,it's cheaper and will look good if made well.
Leather sayas turn up,but really it's a hit and miss,finding one that fits.
I personally think leather sayas are excellent and yours definitely had one, the saya is textbook for leather covering.
The habaki looks an older type,than many ww2 swords I have seen,but I don't know, as the smith is unfamiliar.
The label you refer to is a sword part shop,they appear on painted sayas sometimes.
I hope you get the saya covered, and the sword looks decent.
Good luck and best regards.
Ian Bellis