Go ahead and reskin them, Kevin. They are no doubt nice instruments, but not out of the ordinary, and should be played rather than just looked at, I think.
The things you describe - the seamless joint work, and the soft wood end pieces for the neck joints (called aigi) are standard on a decent shamisen. A higher grade shamisen would have a herring bone pattern inside the dou, and metal fittings in each of the neck joints (rather than the standard wood-on-wood).
The writing in the pictures is hard to read for sure - the stamps are too faint, and the handwriting is not all that clear either, but here are a few things I picked up:
First picture: the far right column says - “33rd year, 7th month” (of the Showa era meaning 1958), the year it was made, or, more likely, last skinned. The next column is the name Miyamoto, probably the customer’s name. The blue stamp is the business name and address. Hamaki is the shop’s name, and they deal in koto and shamisen. The address is too blurry to read, but the last line contains the word Sannomiya, which might be the Sannomiya district in Kobe.
Second picture: The name Miyamoto again, but I can’t make out the second line.
Third and fourth pictures: show the label with the shop name and date of work done on the second shamisen. The customer’s name is Uehara, and it looks to be done by a different shop, but it’s too faint to read.