1. Those made prior to 1879, which will usually, but not always be cancelled using a Lagos diamond grid killer.
2. Those made after 1878, which will usually be cancelled with a barred oval grid consisting of either 8 or 9 bars.
Thus, where there are 3 or fewer printings, of a stamp, these characteristics alone will generally be sufficient to assign stamps to a particular dispatch with relative confidence. However, where there are more than 3 printings, the assignment of these to specific dates becomes much more conjectural. Shade differences as well as relative quantities are used to postulate which printing a stamp is most likely to be from, though assigning with certainty is nearly impossible without covers that bear dates that can be used to corroborate the assigned dates already proposed.
So without further ado, let us take a look at the 1d lilac.
First Printing - Dispatched May 9, 1876
The first printing is the most common of the four printings, consisting of 400 sheets, or 24,000 stamps. This printing is in a pastel shade of bluish lilac, which can also be somewhat reddish. On all the stamps that I looked at, the head plate and duty plate (words "One Penny") were the exact same shade, suggesting that they were printed in one operation, from one batch of ink.. Used examples will nearly always be cancelled with the Lagos diamond barred grid killer. The gum on mint examples is very thin, and colourless, with a satin sheen, just as on the perf. 12.5 issue. Used examples generally will show fine vertical mesh on the back.
Below is a scan showing the front and back of a mint example from my stock:
Second Printing - Dispatched June 12, 1877
This printing is the second scarcest of the four, consisting of 206 sheets of 60, or 12,360 stamps. It is the most distinct of the four in that the shade is an unmistakable, deep, bright reddish lilac. The duty plate is also a slightly different colour, being just a touch duller than the head plate ink. This suggests that it was printed in two operations, from two similar, but different batches of ink. Like the first printing, the stamps were all line perforated, and the paper is a coarse wove, showing very distinct vertical mesh.
Unfortunately, I do not have any used examples in my stock, but I do have an unused example:
Now, for the back:
This printing consisted of some 305 sheets, numbering 18,300 stamps. The stamps of this printing are a much paler lilac than the first printing, and are actually very close in shade to the last printing. All of the stamps of this printing are still line perforated, despite the fact that the comb perforators had become available a year earlier. Used examples will have either a Lagos barred diamond grid killer, or a barred oval killer. Again, the duty and head plate shades are an exact match, suggesting that these two were printed in one operation from one batch of ink. The paper used for this printing was a thicker, smooth wove that shows no distinct mesh, and on which the watermark is barely visible. The gum on mint examples is thicker and starts to show a very fine crackly pattern. This is because De La Rue, in response to complaints received from the postmasters that the stamps were not adhering properly began to increase the thickness of the gum applied to the stamps. This would continue on the 1884-1886 issue, before the chemical makeup of the gum was changed completely in 1886 and became the gum that most collectors commonly associate with the keyplate stamps of De La Rue.
Below is the front and back of a mint example from my stock:
Fourth Printing - Dispatched November 18, 1880
This last printing consisted of 200 sheets of 60, or 12,000 stamps. It is easily identified by the fact that the perforation is the comb, rather than line perforation that was produced by the new comb perforating machines installed by De La Rue in 1878.
The shade of this stamp is a pale milky lilac, similar to the first printing. Furthermore, on all the examples that I examined, the duty plate and head plate are the exact same colour. The paper varies from a thicker white wove, showing no mesh that is very smooth, to a coarser wove that shows a fine mesh. The watermark on this first type of paper is visible, but feint, whereas, it is very clear without watermark fluid on the second type of paper.
Below are scans of both the front and back of a mint example from my stock: