I also infer that the screws must have tended to loosen on the Hilgen knobs, because it is very common to see an assortment of different knobs on the same amp or a complete replacement of the original knobs with vintage but non-original types.
In any event, I first noticed that the Sano amps produced during the transition period used the same knobs Hilgen later used. Then I noticed that the "Sano" brand name was not always included on the black control panel. Finally, I noticed a Sano for sale on eBay that had Hilgen knobs, a Hilgen-style back-panel, and a control panel that looked almost exactly like the control panel on the smaller Hilgen black-face amps, minus the Hilgen name. So, I recklessly bought the amp to see what's inside it. I wonder how similar the circuits and components will be....!
By the way, some Hilgen amps did go through a transition from "black-face" to "silver-face" control panels at about the same time that Fender did. I know that for a fact with respect to the Troubador model, having personally seen a chrome control panel on a 1967 model and a black control panel on a 1965. Because I do not own a pair of Hilgen models in both black-face and silver-face versions, I do not know if the cosmetic change was accompanied by any changes of circuitry. I doubt it. But it would be hilarious if some day people start arguing over whether the black-face Hilgens are "better than" the silver faces. Now that would be hype! (I think.)