A CVR is standard equipment on Series 400 Twin Otters, every one that ever left the factory was equipped with both a CVR and a FDR.
My statement that DHC-6 doesn't have them was incorrect. CVR/FDRs on the -400 (only) may have been fitted initially (as you said, you wrote the manual so you also probably also reviewed and memorised every single equipment list on every -400 produced) but I have yet to fly one that
still had them installed.
CAN BE FITTED I believe is the exact wording used in the -400 AFM for both devices.
As noted above, there is no factory certification of any kind for overweight operations.
Yep. My bad. Good job clarifying in case someone went and tanked a Twotter based purely upon my statement.
(BTW, Something in inverted commas/quote marks, such as "Factory Certified" normally can be taken to be a generalisation.)
https://media.giphy.com/media/qs6ev2pm8g9dS/giphy.gif
My comment was more directed at the fact the Twin Otter is one of the few types IME that actually published a ferry system in the AFM (B-N Islander is another). And (as you wrote it) the DHC-6-400 does mention this in Section 7 Systems in the AFM as well as further information in a supplement.