A large number of the other props and products used on the show are not consistent with the period (flip-top drinks cans, barcodes on toys, etc, etc) or are not represented accurately (Betamax VCRs that could record too much material on one tape, etc).
In the opening scene of the premiere episode, the boys are looking at a Playboy centerfold, circa 1976. However, the magazine is perfect bound, not saddle stitched (stapled) as Playboys were at that time.
In the Show, Eric turns 17 years old in 1976, placing his birthday in 1959. However, the crew doesn't graduate High School apparently until 1979, placing all of the characters at 20 years old at time of graduation
Throughout out the series (esp. during the last seasons), it's clear that most of the actors/actresses portraying teenagers are actually in their 20s+. This is done as to not interfere with the schooling of real teenagers and to give the general viewing audience who are teenagers (or young people) themselves someone they can look up to.