Since the W108 and W109 were only available as 4-door models, similarly squarish 2-door W111 and W112 coupés and cabriolets, without pronounced tailfins, filled those niches and are frequently mistaken for W108/109 two-doors.

Unifying the entire Mercedes-Benz range into the fintail (German: Heckflosse) reduced production onto a single automobile platform.

By the time the Paul Bracq-designed 2-door coupé and cabriolet W111s were launched, the predecessor W111 sedan's fins lost their chrome trim and sharp appearance.

Although the fins' departure was the most visible change, the W108 had a lower body waist line and increased glass area - the windscreen alone was 17% larger than W111's - prominent enough to be referred to as a "greenhouse".

The larger 3-litre six-cylinder inline engine (300 SEL) were exclusive to W109 with odd exception of standard wheelbase 300 SEb (1965–1967), which was designated as W108.

W109 was more luxuriously appointed than W108, featuring burled walnut dashboards, automatic transmission, and power windows along with optional air conditioning system.

Most obvious distinction from the W108 were door window frames and a-pillars fully finished with polished metal bright trim.

The 250 S was fitted with 2,496 cc M108 engine with dual two-barrel downdraft carburettors producing 96 kW (131 PS; 129 bhp) at 5,400 rpm.

With increased engine output of 110 kW (150 PS; 148 bhp) at 5,500 rpm, 250 SE was one second faster to 100 km/h than 250 S and had higher top speed of 193 km/h (120 mph) for both manual and automatic versions.

The tiny production figure of 300 SEb/300 SEL reflected the higher sales price with luxurious appointments, exclusivity, and smaller global market share.

To coincide with the launch of new cheaper executive, W114/115 "/8" (Stroke Eight) models, slotted below MB's W108 and W109, the inline six engine range was revised in 1967.

Following the strong reception of limited production V8-powered 300 SEL 6.3 in 1967, Mercedes-Benz offered the all-new 3.5-litre M116 V8 engine with new Bosch D-Jetronic electronic fuel injection in 1969.

The 3.5-litre V8 engine was bored out to 4,520 cc (4.5 litres) as to compensate for the loss of power following the changes in emission control regulations and due to lower fuel octane rating in the United States.