Tom Archer wrote:The Classic Declaration as we know it today is, largely, a post WW2 creation.
I would argue that the modern era of vintage declarations started in 1896. The pattern was pretty consistent between then and 1960, if you allow for the cancellation of the 1937 declaration due to the outbreak of WWII, and 1942 being poorly attended for the same reason.
After 1960 the pace of declarations increased from around one every four years or so to the three times a decade we see today.
So like I said, largely after WW2.

Remember the requirement to bottle between the second and third year after harvest didn't come about until the late 1940's. If you search on here there is a thread where Paul Symington talks about when his father was on the committee that came up with that new rule. Producers bottled/declared when they saw fit, and for some that could be a number of years later than others.
In reality, prior to WW2 there was a lot less regulations for certain things than now. Also keep in mind the top shippers today were not the top shippers back then. Cockburn's was generally the most expensive VP to buy, not anything from TFP/SFE (as we know them today). So TPF/SFE were not the "shot callers" for any type of declaration like they are today. Producers made a VP when they had a good product, which wasn't all that often when one had to rely largely on mother nature. Unlike today where technology allows one to make a darn good VP even in an average year.
I do agree with you that around the early 1960's there was a definite turning point, but again that was driven in part by more advanced technologies and newer regulations. For example, Dow's got their autovinifiers up and running for the 1963 harvest and used them until the early to mid 2000's IIRC (This is a fun one to bring up when people say autovinifiers don't make good VP....1963 Dow's, and subsequent vintages, are pretty amazing).
EDIT: Here is what Paul Symington said....great reading for those who haven't seen it.
There is more misinformation about LBV than just about any Port category, with some wild claims made to say the least. The real story is that for centuries the actual bottling time for Vintage Port was not defined. The producer would decide when he wanted to bottle, sometimes aftet two years in cask, sometimes after 4 or even 5 years. When the IVDP decided, shortly after the II World War, that the bottling times should be defined, some producers pushed for two years but some pushed for longer. My father, Michael, just back from the war, was working with his father Maurice and his uncles as a Port producer in our company in 1948, and he was actually on the committee that decided this (called the Gremio at the time, now the AEVP) with the IVDP. The compromise solution found was that Vintage Port would be bottled in its 2nd year and that a new category would be formed, called, very logically; Late Bottled Vintage Port. I believe that the first such wine was Ramos Pinto (who would certainly have had older wines that could use this new designation as they historically bottled their Vintage Ports later). My friend Joao Nicolao de Almeida from Ramos Pinto can certainly confirm this for you. Obviously the wine was supposed to be bottled un-filtered and un-fined, just like Vintage Port. We ourselves still have bottles in our cellar of Dow's 1964 LBV and Warre wines from the 1950's made in this way and duly registered with the IVDP. At a later stage Cockburns decided to launch a filtered LBV, in an attempt to widen the Port market that was very very difficult at that time, but the outcry from all the major Port houses was such that they rapidly withdrew the wine. Peter Cobb, retired Director of Cockburn's, can confirm this fact for you. Later Taylor's introduced a filtered LBV that became a considerable commercial success in the UK and credit to them for having done so. We later introduced a Graham LBV and so have many others. These wines have become one of the mainstays of the Port trade (and of course for the Douro) and are essential to its survival. Incidently, with our Warre's LBV we have always stuck to the original intention: 4 year bottling, no fining or filtration, driven cork and above all, we mature it in bottle for at least 3 years before release. A different (and inevitably more expensive) LBV. But both styles of LBV have a rightful place in any Port lovers cellar, they are both excellent wines.