The IMP spacecraft series. Prelaunch, the 10 IMP's were
called IMP's A-J. 8 of them were intended for geocentric orbits
(A-C, F-J) and 2 for selenocentric orbits (D,E). Prelaunch, the
latter two were also called Anchored IMPs (AIMPs) A and B.
After launch, the 8 intended for geocentric were also called
IMPs 1-8 and the 2 intended for lunar orbit were called
AIMP 1 and 2. Only IMP E = AIMP B = AIMP 2 achieved lunar
orbit; IMP D = AIMP A = AIMP 1 went into a geocentric orbit.
The mapping of prelaunch IMP letters to postlaunch Explorer numbers
and IMP numbers was

      +-----------------------------------------------+
      | IMP (prelaunch) | IMP (postlaunch) | Explorer |
      |-----------------------------------------------|
      | A | 1 | 18 |
      | B | 2 | 21 |
      | C | 3 | 28 |
      | D |   | 33 |
      | E |   | 35 |
      | F | 4 | 34 |
      | G | 5 | 41 |
      | H | 7 | 47 |
      | I | 6 | 43 |
      | J | 8 | 50 |
      +------------+