I assume you mean a 1966?
All five speeds that left the MB factory as such were ZF gear boxes. All 5 speeds that are around now are not necessarily ZF or if they are ZF are not the original gear box in accordance with the data card but have been modified either from another W113 but most probably from a sedan. There are probably more 5 speeds in the US with ZF gear boxes that ever left the factory with them for that market. If it was RHD you could probably say it was original, LHD from Europe probably also ok but a LHD from US maybe 50/50.
The best way to know if it is original is to see the data card and what gearbox it left the factory with and establish if it has been converted or not and to what.
They do bring a premium if it is a genuine original and not a conversion. The converted cars are IHO probably worth a bit extra to cover what the gearbox cost to buy and fit but that is about it.
If you can get an original go for it, they are a very very expensive gearbox if they go wrong and at this point as there are next to no spares available but there has been some rumours of ZF producing spares at some time in the future.
