A039326 Numbers n such that representation in base 7 has same nonzero number of 4's and 5's.

33, 39, 82, 88, 131, 137, 180, 186, 201, 208, 215, 222, 231, 232, 233, 234, 237, 243, 249, 256, 263, 270, 273, 274, 275, 276, 279, 291, 327, 333, 376, 382, 425, 431, 474, 480, 523, 529, 544, 551, 558, 565, 574, 575, 576, 577, 580, 586, 592, 599, 606, 613