To complicate things further there's also a MK I-B, a transitional Vacumatic filler with the new style non-blue diamond cap with a long clip.
The MK II can also be divided into MK II-A, with the longer clip cap, (identical to the MK I-B), the MK II-B with a shorter clip and MK II-C with an even shorter clip with fewer feathers and the "halo" logo on the cap.
The Parker "51" Mark II is probably the most functional fountain pen ever made.
It never leaks, always works and never breaks, well almost...
There are however today reproduction jewels available in aluminium, so this feature alone is not a guarantee that the pen is a First Year. It's not uncommon for the First Year pens to differ in the colour shades from pens later produced. There's been some confusion, since David Shepherd chose in this book "Parker 51" not to count the Vacumatic filler as , instead starting with the first aerometric model.
Furthermore these pens were fitted with clips that had a larger blue diamond than subsequent pens and if the clip is removed the Parker imprint is visible in reverse on the back, on accont of a different manufacturing method. It might become grounds for confusion, but In this article the Vacumatic filler will be referred to as the MK I, the Aerometric with the rounded end as the MK II, the aerometric with the plastic jeweled "61" style cap and squarish end as the MK III and the aerometric with the metal jeweled "61" style cap as the MK IV.
The replicas do not (so far) carry any date code imprints.