The ultramodern looking stereo unit briefly seen at about 24 minutes playing Ludwig van Beethoven's 5th is a Nakamichi 700 cassette deck, one of the two first true high fidelity and 3 head cassette decks made (Nakamichi 1000 was the other).
The Moonbuggy vehicle gets pretty battered in this episode, possibly explaining its final appearance on the series here.
Koenig's line "We've been in space for months, which in Earth terms is generations", as well as Helena's status reports announcing how many days they've been away from Earth, tactfully avoids using the word "years", which would discredit the very title of the show.
The location footage was filmed on the grounds of Pinewood Studios. The canoe Alan was paddling was filmed inside the small body of water Koenig and Helena had investigated on The Full Circle (1975) where the large animal print had been found.
The stock aerial footage of lower Manhattan (when Alan thinks he's descending to "earth") likely dates to 1972 or 1973 as evidenced by both 2 World Trade Center and 130 Liberty Street being under construction. Both buildings were lost as a result of the 9/11 attacks.