So here's the deal from my standpoint. There's little point to the traditional three era - Pre-Transition, Transition, Post-Transition - set of definitions. Here's why.
The simple breakdown goes like this:
Pre-Trans Barling - Barling pipes made while the family owned the company. Depending on your source this period extends from 1812 to 1960, or 1962.
Trans-Barlings - Barling pipes made after the family sold their business to the "transition" company. This period, depending on who gives the dates, extends from 1960 or 1962, to 1964, 1968, etc, etc.
Post-Transition - Barling pipes made after the "transition" company was bought by Imperial Tobacco. This era extends from 1964 or 1968 thru the present.
It's nice. It's simple. And, it's bullshit.
So, here's what happened.
The Barling family owned and ran their business until October of 1960, when they finalized the sale to Finlay, which was their largest customer and which ran a large chain of tobacconists throughout Britain. At the time that Finlay bought B. Barling & Sons, they were themselves 49% owned by Imperial Tobacco.
Even though the Barlings sold their business, they were retained by Finlay to run it. Montague Barling remained as president of the company, and Williamson Barling remained the general manager. The staff remained essentially the same. Even the pipes were made in the same manner, by hand, and the nomenclature remained exactly the same as when the family owned the business. This state of affairs continued for roughly 20 months after the Barling family had sold their business to Finlay, the so called "Transition" Company.
So although the "transition" era began in late 1960, there's no way to discern any difference between the last "Pre-Transition pipes and the early "Transition" pipes because the stampings workmanship and materials are the same. That's 20 months of production that cannot be distinguished.
In 1962 things get interesting.
The Barling managed/Finlay owned company started to change the stampings. Pipes were stamped with the 1930's era "Barling's Make" stamp and also with a model numbering system that used a 4 digit number whose first digit represented the chamber size. First digits ranged from 2 thru 6. These new stampings first appeared in print in the 150th anniversary catalog that was issued in mid 1962.
In June 1962 Williamson Barling was given the sack and not much later Montague stepped down. Finlay placed their own people in charge and began to make changes in the operation to expand the range of products, and to make pipes for others, notably Falcon. Many of Barling's senior carvers were let go. This was an incremental process.
By November of 1962 the product line had been completely revamped and the nomenclature had been changed. Gone was the "Barling's Make" logo, stamped in block letters with the word "Barling's" arched over "Make". In its place was a script "Barling" with the words London England stamped beneath the new script logo. The new model system was retained. These pipes are identified as either "Transition" or "Post-Transition" because most people can't tell the difference. In November of 1962, Barling issued a new catalog with the new stampings, finishes and new gradings.
The "Post_Transition" era is supposed to begin in either 1964, or 1968, depending on which source you consult as that's supposed to be when Imperial Tobacco bought the "Transition" company (Finlays). In point of fact, all of that is wrong. Imperial already owned a 40% stake in Finlay and had an option to buy the rest of the company. This they did in February of 1963.
So, you have a "Transition" era of 2 years and 4 months and you have 20 months of production that can't be identified as to era. The so called "Transition" era might as well not exist. It's effectively meaningless.
Once Imperial took over, another management shuffle took place and the pace of change accelerated. In 1968 Imperial closed the Barling factories and outsourced production to Charatan and the Ogden group, as well as other companies. Later, Imperial contracted with Nording to make Barling pipes for them. In 1980, Imperial sold the Barling name and rights to an investment group who set up operations on the Isle of Man with intention of restoring the Brand to a high level of quality.
As to the fabled Barling wood, there's a bit of a story there as well. In 1906, Barling decided to make all of their own bowls rather than make some and import others from Saint Claude, which was common practice. They decided to control the quality of the wood from the ground to the sales counter. To do that Barling set up their own harvesting operations in Algeria and did their own cutting, aging, etc.
This state of affairs continued until 1954, when a little thing called the Algerian War of Independence put an end to Barling's operations. After 1954 Barling sourced from anywhere that they could find briar that met their quality requirements.
There are Barling collectors who say that the "Pre_Transition", "Transition", "Post-Transition" breakdown is wrong because it ignores the change in the material used to fashion the pipes. Their breakdown is as follows and it is also useless. 1812-1954 - Pre-Transition; 1955-1962 - Transition 1; 1962-1968 - Transition 2; 1968 onward - Post Transition.
The simplest and most accurate way to break down Barling eras is Family Era, which encompasses all pipes made while the family either own or ran the business, and Corporate era, which would start with the change of nomenclature in late 1962, and encompass all of the changes that have taken place since. It's not perfect, but it's one hell of a lot more accurate.
And now you know the difference, and why it doesn't actually matter.