In 1984, the SRS system (ETR's and drivers side airbag) was an option on the W126.
In 1985, ETR's became standard equipment on the 107, 123, 126, and 201, except model 123.153. I know the airbag was available in a 107 chassis, so I am kind of assuming at this point that ETR's were standard and the airbag was an option on the 107 and 126 in 1985.
In 1986 forward, all Mercedes came standard with dual front ETR's and drivers side airbag.
In 1988 they "unitized" the SRS control unit to include the crash sensor, energy accumulator, and voltage converter in a single housing.
In 1989, passenger side airbag became standard on 126.035, 039 and 045, and optional on 124, 126.024 and 025.

What they are concerned with on older airbags is degradation of the airbag cloth.

Gilly