A039420 Numbers n such that representation in base 10 has same nonzero number of 3's and 7's.

37, 73, 137, 173, 237, 273, 307, 317, 327, 347, 357, 367, 370, 371, 372, 374, 375, 376, 378, 379, 387, 397, 437, 473, 537, 573, 637, 673, 703, 713, 723, 730, 731, 732, 734, 735, 736, 738, 739, 743, 753, 763, 783, 793, 837, 873, 937, 973, 1037, 1073