ETK 2400i

6- speed manual from the factory just wasn't a thing back in the 80s. The homologation cars and the Knallhart use race transmission. The 5-speed's 1st gear doesn't have the same ratio as the reverse gear, the 1st is 3.72, the reverse is 3.45.

 

Pretty sure nothing in Beam is entirely based on IRL lore, its whatever they decide the lore is. So in BeamNG lore, there was indeed factory 6 manuals in the 80s

 

There've been (very limited) production cars with six-speed manuals as far back as the 1960s IRL, with the original Alfa Romeo 33 Stradale (18 produced). 6-speeds started becoming more common in cars in the early-1990s, with cars like the BMW 8-Series (1990), Ferrari 456 (1992), and Toyota Supra Mk4 Turbo (1993). So, it's entirely possible for a racing homologation version of a car built in the late-1980s (like the I-Series Evo) to receive a 6-speed manual.

 

And more importantly, a 6-speed transmission swapped in would have a different shift pattern, even if the shifter itself looked the same outwardly.