There must be a flaw in my logic, but I can't see it. Any help gratefully accepted: 4d/19d must be 50, 52, or 70. If 19d ends in a zero, so does 21a which must give 370 but this is impossible since 15d requires the middle digit to be 0, 2, 4, 6, or 8. So 19d is 52 and 4d can't be (no two entries the same) so 4d ends in 0 and the middle digit of 8a is 0, which is impossible since the second part of the entry can't start with 0 and the first part must be prime...