While the concept of the automotive "model year" is now firmly established in the public mind (due to incessant marketing by the car companies), it was not always so. In the early years of Porsche, the company was mostly indifferent to the concept, and corporate record keeping was focused on production years. Significant changes to the Type 356 usually occurred when production resumed after the summer break (which was typically in August) but changes could also occur at any time, such as the change to teardrop style taillights in March 1957 or the increase in the 356A front overrider tube height in January 1959. Kardex records for the Type 356 do not contain any model year information (the Kardex often shows the date the car was shipped from the factory or when it was received by the owner / dealer) and 356 factory literature rarely refers to "model year".
This indifference to model year is also observed in British manufacturers of the period, such as Triumph and MG. The concept of a model year may have been initiated by American automobile companies as a way to motivate consumers to buy new cars more frequently, even if the only change from one year to the next was bigger tail fins and more chrome trim.
As the American market grew increasingly important to Porsche in the 1950's, perhaps the company realized that there was some sales advantage to be gained by emphasizing a "new" model every year, even if the cars hardly changed. For whatever reason, sometime in August 1958 a new plaque appeared next to the large chassis number plaque on the trunk floor next to the gas tank on the passenger side, as shown in the photo below. It read "MODELL 1959 MADE IN WESTERN GERMANY". Prior to that time, the plaque simply read "MADE IN WESTERN GERMANY". Despite this designation, the 356A essentially did not change between July 1958 and September 1959, though the floor mats in the coupes and Cabriolets were changed from black to tan for the 1959 model year (1959 model year Convertible D's had black mats when the bodies were shipped from Drauz to Porsche: of course mats could have been changed by dealers if the customer requested it).
Note that the "Baujahr" (meaning build year or year of manufacture) section of the chassis number plaque was sometimes stamped with the year, but often left blank.
The earliest coupe chassis number that we are aware of with the MODELL 1959 plaque is 104660 which was built in September 1958 (the highest numbers known to not have such a plaque are 104387 and 104869, but keep in mind that the cars were not built in the same order as their chassis numbers). The earliest Cabriolet reporting having the plaque is 150950, production date 8/27/58 (per Steve Heinrichs). Many coupes, Cabriolets, and Convertible Ds built during or after September 1958 have reported having the plaque, such as coupe 104737 which was built on 9/17/58. And here's a puzzle: Freddy Rabbatt owns coupe 107330 (build date March 29, 1959) which has the "MODELL" plaque and Cabriolet 151925 (build date April 30, 1959) which does not have the "MODELL" version of the plaque. Speedsters built before September 1958 do not seem to have it.
As of this writing, several T5 356B cars have reported having a plaque that reads "MODELL 1960 MADE IN WEST GERMANY" (photo at right courtesy of Eric Cherneff), including Roadster 86904 (built 10/02/059), coupe 110249 (built in early January 1960) and coupe 110471, and Cabriolet 153311.
One "MODELL 1961" plaque has been reported for Cabriolet 153888 (see photo below) Strangely, that chassis number is for a 1960 production year and model year car. We are not aware of any T6 cars—which went into production in late August 1961—that have the "MODELL" plaque showing the year. Their plaques simply say "MADE IN WESTERN GERMANY" (no "MODELL" year is shown).
If your 356 has a "MODELL" plaque that shows a year, please contact firstname.lastname@example.org and provide the chassis number and build date if you know it.