It's probably on a ship somewhere in the Pacific. Don't give up hope. My bet is you will eventually receive it, but "when" is the question. Obviously, anything going from Singapore to NH, or anywhere else in the US, is going by air. Obviously, your Koetsu isn't. I wondered why you chose to do the rebuild with "Koetsu SE". Koetsu, the parent company, is in Tokyo. I imagine Koetsu SE is a distributor, only. In my experience, the parent company in Tokyo are very fastidious about service and ship by air. (I purchased an Urushi in Tokyo, from a bona fide Koetsu dealer who gave me a certificate in Japanese. When I got back home to Bethesda, MD, I found that the cantilever was misaligned. I sent it back to Tokyo, and they replaced the defective sample with a perfect one, no questions asked. The whole transaction took no more than a month.)
And let's not blame the USPS. So far, the USPS has had nothing to do with shipping this cartridge and won't until it finally arrives at the OP's local post office, whence it would go out for delivery. USPS is too often unfairly the whipping boy.