This is definitely a fine mystery! It's not arguing, it's lively, welcome discussion. I often learn new things or find better rabbit holes from it.
First, do you have a picture of the mechanisms you described in the post ahead of leonf's?
If Necchi took over a mfg contract, they would have had to build in the Viscontea factory as well, I think. Otherwise they would have to move the tooling, and have the physical space and production capacity to do that. The machines would look like Necchis and be engineered like them if they switched to their own facilities.
Sewing Circle had a chain of stores similar to Singer's. They originally imported Necchis and added the US current-correct motors to them. Later, Swiss-made Elna machines (also often green!) were made available their network of shops. Part of what sunk them was an over- ambitious plan to sell their own private label, Japanese-made budget Nelco machines. The other suppliers took umbrage and sued over design and trade name issues, and it kind of fell apart from there. I read once that the motors were available from the stores, and could be fitted to other machines by their service department. Also, Sewing Circle was the US distributor. It was less expensive to fit the US motors here than there.
There are some similar plain-faced, squarish Necchis (BF & BU). My recollection, though, is that the pillars are narrower and the guage slider is more centered.