The pennants in question are not specific to Model, but rather specific for an officer of his rank and command. The pennant on the passenger's side of the car is Hoheitszeichen am Kraftfahrwagen für Feldmarschälle (field marshal's emblem for motorized transport). The red-banded checkerboard flag on the driver's side is a Flagge für den Oberbefehlshaber eines Armeeoberkommandos (command flag for the c-in-c of an army group). These types of command pennats were not that uncommon, see for example this 1940 photo of Walther von Brauchitsch. The pennants though were much rarer as Germany lost both air superiority and partisan activity picked up. As for the film's encyclopedia and identification, this is likely a narrative convention. Cornelius Ryan's A Bridge Too Far has this to say about the Dutch underground's attempt to find out the new occupants of the Tafelberg Hotel:

Twenty-five-year-old Wouter van de Kraats had never heard of Henri Knap. His contact in the underground was a man he knew only as “Jansen” who lived somewhere in Arnhem. Jansen had a new assignment for him—the Tafelberg Hotel. A high-ranking German officer had arrived, he was told, and Van de Kraats was to see if any of the staff cars outside “carried an identifying pennant or flag.” If so, he was to report the colors and symbols on the standard. Van de Kraats had noticed an influx of German activity around the hotel. German military police and sentries had moved into the area. His problem was how to get through the sentries along the road—the Pietersbergweg—running past the Tafelberg. He decided to bluff his way through. As he made for the hotel, he was immediately stopped by a sentry. “But I must get through,” Van de Kraats told the German. “I work at the petrol station up the street.” The German let him pass. Three other sentries gave him only a cursory glance. Then, as Van de Kraats passed the Tafelberg, he quickly looked at the entrance and the driveway. None of the parked cars had any identifying markings, but near the front door of the hotel stood a checkerboard black, red and white metal pennant—the insignia of a German army group commander.

The encyclopedia scene was likely taken from another incident recorded in the nonfiction book:

On the afternoon of Thursday, September 14, {Dutch resistance leader} Henri Knap heard from his network. Several sources reported large formations of panzer troops, tanks and armored vehicles encamped in a semicircle to the north of Arnhem. There were units at Beekbergen, Epse and along the Ijssel River. There was even a startling report of “20 to 30 Tiger tanks.” Exactly how many units were involved, he was unable to ascertain. He was able to clearly identify only one, and that by a fluke. One of his agents noted “strange markings—reverse F’s with a ball at the foot of them”—on some tanks. Checking through a special German manual, Knap was able to identify the unit. He immediately called his telephone contact and reported the presence of the 9th SS Panzer Division Hohen-staufen. From the agent’s report, Knap located its position as lying approximately to the north between Arnhem and Apeldoorn and from there, eastward to Zutphen.

Ryan then goes on to note that Knap received Van de Kraats's report and that all he needed was the checkerboard to know that this was an army group commander and there was only one army group commander in the Netherlands: Model.