Don't hold out hope of getting a true sense of the linage of your boat; but please share anything you learn... Hopefully the below will help some:
There were a series of (what I believe are) DeFever copies/lookalikes made in Taiwan in the late 1970's to late 1980's that had a distinctive look to them. Most were, like the photos from Menzie's, drafted along the lines of DeFever's Passagemaker. They may have been directly from DeFever molds that were left in Taiwan, or pulled from DeFever boats, or original designs intended to be copies, I have no idea. They are remarkably similar, I think. Whatever they are, they, like the Passagemaker, have beautiful lines - that's what we fell in love with when we found NWD.
There was a specific hull mold that seemed to generate the same look/interior as your boat (and mine.) They are an approximate 41 ft LOA / 14 ft beam semi displacement hull, with a single chine that is not as hard/flared as a Grand Banks would be. The keel starts at the bow and continues straight back nearly to the props. The keel is molded for a single, but many boats were made with twins.
The boats nearly always have the twin portholes in the v-berth, have the same uniquely shaped anchor locker doors, as well as similar cabinetry throughout the years. It appears the same craftsman/families continued to have a part in building the boats since so many things stayed the same, despite the passage of time/different "pedigree".
Just like CHB/Marine Trader 34', these boats I fear suffer from various pedigrees. However, my boat too was built in Keelung by Seiyen Enterprises.
I've found them under the name Davis International Yachts (most examples), Hudson, Lindmark, and Tien Lai Ling. They are often marketed by brokers as one of those names with "DeFever" attached, even though I have yet to find DeFever's name legitimately attached as designer anywhere. I have copies of both an original sales brochure from Davis International for my boat, as well as magazine ads, and neither mention his name. I believe mostly the above companies were importers who found the boat builder and decided to "make it there own". There were a few differences. For example, Lindmark (I think) was the only one that had all teak decks - all the rest had fiberglass non-skid decks
The boats also bear a distinct resemblance to the Formosa (of the famous sailboats) 42 ft trawlers.
The later 1980s (and even early 1990's) boats have a portion of the lazarette pushed back to make a walkaround queen, as well as the rest of the aft deck/lazarette sunken to create a mini-cockpit area. Obviously these boats had a change to the cabin top mold. Otherwise, aside from different engines/appliances, they look remarkably similar in and out.
While I don't know any more about your boat (it was on the market for a long time as a "Davis DeFever". Davis Marine WAS in business until 1988 according to WA state records... but I don't see the Davis nameplates on the sides of the flybridge, I also have a large one on the transom...) I have found examples of similar boats marketed as 41 ft Unison DeFever's. Those were 2001 models.
With all the other documentation I received from my PO, along with the original engine warranty cards, etc, I received (very basic) original wiring and plumbing diagrams labeled "Unison 41" and labeled with several numbers matching my HIN. So I believe that the "Unison 41" is the original common name/design, but don't know if that is a company or design name. The Unison company exists in Taiwan today only as a shipping/steamer company.
I've never found a builder's plate on my boat.
Hopefully this is a start for you. If you'd like more info, or some of the files/data I've gathered, let me know.
Congrats on the boat, I bet the cockpit makes life a dream getting in and out of the dink!
