VW t1 deluxe split bus. 23-window (left) and 21-window (right)
The early versions of the T1 until 1955 were often called the T1a or "Barndoor", owing to the enormous rear engine cover, while the later versions with a slightly modified body (the roofline above the windshield is extended), smaller engine bay, and 15 in (381 mm) wheels instead of the original 16 in (406 mm) ones were called the T1b. From the 1963 model year, when the rear door was made wider (same as on the T2), the vehicle was referred to as the T1c. 1963 also saw the introduction of an optional sliding door for the passenger/cargo area instead of the outwardly hinged doors typical of cargo vans. This change arguably makes the 1963 VW the first true minivan, although the term wouldn't be coined for another three decades.
t1 split bus, camper van (left) and 11-window barndoor (right)
In 1962, a heavy-duty Transporter was introduced as a factory option. It featured a cargo capacity of one metric ton (1,000 kg) instead of the previous 750 kg (1,653 lb), smaller but wider 14 in (356 mm) wheels, and a 1.5 L, 42 DIN hp (31 kW) engine. This was so successful that only a year later, the 750 kg, 1.2 L Transporter was discontinued. The 1963 model year introduced the 1493 cc (1.5L or "1500") engine as standard equipment to the US market at 51hp (38 kW) with an 83 mm (3.3 in) bore, 69 mm (2.7 in) stroke, and 7.8 to 1 compression ratio. When the Beetle received the 1.5 L engine for the 1967 model year, its power was increased to 53 hp DIN (40 kW).
German production stopped after the 1967 model year; however, the T1 still was made in Brazil until 1975, when it was modified with a 1968-79 T2-style front end and big 1972-vintage taillights into the so-called "T1.5" and produced until 1996. The Brazilian T1s were not identical to the last German models (the T1.5 was locally produced in Brazil using the 1950s and 1960s-era stamping dies to cut down on retooling, alongside the Beetle/Fusca where the pre-1965 body style was retained), though they sported some characteristic features of the T1a, such as the cargo doors and 5-stud (205 mm bolt circle) rims. Brazil production air-cooled vehicles (including the VW Brasilia) are a rare find in the USA and usually sought after by collectors.
Among American enthusiasts, it is common to refer to the different models by the number of their windows. The basic Kombi or Bus is the 11-window (a.k.a. 3-window bus because of three side windows) with a split windshield, two front cabin door windows, six rear side windows, and one rear window. The deluxe model featured eight rear side windows and two rear corner windows, making it the 15-window (not available in Europe). And the sunroof deluxe with its additional eight small skylight windows is, accordingly, the 23-window. From the 1963 model year, with its wider rear door, the rear corner windows were discontinued, making the latter two the 13-window and 21-window respectively.