I'm not so sure about that. Mind that they talk about household income, not individual salaries.

The median personal income in 1975 has been $26,450 [1], although that's across all generations (not sure if your number is).

Edit: Unfortunately the statistics about the median household income [2] don't date back that far.

[1]: https://fred.stlouisfed.org/series/MEPAINUSA672N

[2]: https://fred.stlouisfed.org/series/MEHOINUSA672N