Of the seven, only two were built outside of the Frontier Closed Area - a strip of land running along the border within which a permit was required to enter (unless you were a villager living inside the area). Given the logistics of having a large TV production stomping around inside the FCA, it's no surprise that the one featured in the show is one of the two located outside of that area (side note: most of the FCA has now been removed, ostensibly to allow easier visitation but in fact the main reason was to facilitate easier redevelopment).
The one we see below is located in Maipo. Wikipedia lists it as being located at Pak Hok Chau (白鶴洲) but actually the area is really Tam Kon Chau (担竿洲) - directly opposite the Mai Po Marshes WWF Visitor Centre. However, it is still known as the Pak Hok Chau Operational Base.
It's nearly 40 years since this show was made so it's not surprising to see a lot of tree growth and development of the roads, but the basic pattern of the surrounding ponds hasn't changed much.
The Fort with Mai Po marshes in the background
If you want an idea of how green and lush the marsh area is these days, feel free to browse a post on my other blog from when I visited a couple of years ago.