This is something of a messy area of stamp production, or should that read interesting. It covers the introduction of die 2 with alphabet 2 check letters as found on C1s and C2s, which promptly runs amuk off the lead with two different perforation gauges and two watermarks. With the introduction of alphabet 3 things generally calm down to a perf 14 - large crown format. But turn your back and they start messing around with the ink colour, and have another go at perf 16. Oh and alphabet 4 pops it's head up on plates 50 and 51.
In order to help me pigeon hole the stamps in my collection I came up with a spreadsheet which I find helpful. I have called this spreadsheet alphabet 3 filter, but it really is more of a die 2 filter. A tool to help narrow down the possibles when you haven't got a memory like MW. Depending on your office suite, the first line on the spreadsheet should appear as filter buttons.
AP
2016-20