Had a lug nut come loose on my truck once, same sinking feeling.

That 1968 spool from Portland is a real find, and the fact that it still stitched clean after 50 plus years says a lot about how linen was made back then. I feel you on wanting to keep it for special projects, because once that spool runs out you are never getting that exact thread again. My grandpa left me a tin of old buttons and honestly I still have not used most of them for that same reason. If the tension held the same as your modern stuff, save it for book work that really matters to you. Just keep it out of direct sun and damp so it stays good for the next round.

Loaned my puller set to a buddy once and got it back with the screw threads all chewed up from him using a impact on it. Now I just tell people the tool box is closed, go buy your own.