In the first series, beginning in 1976, Reginald Perrin is said to be 46 years old, making his year of birth 1929 or 1930, but in the Legacy of Reginald Perrin, on his grave it says he lived from 1924 to 1995, which would make him at least 51 by the beginning of 1976 - unless the original series was made in 1976, but set in 1970 at the earliest.
Reggie explains his late arrival at the office with the excuse "staff shortages at Nine Elms". However, there was no railway or Underground station at Nine Elms at the time. (A new Underground station was opened in 2021.)