The strap is not needed because there is some support of the stereo body along the side and bottom provided by the space M-B uses. A strap is a must for most stereo installations because the radio is mounted at the front with no side support (both DIN and old style with knobs). I used to install car stereos and I have done lots before becoming a manager - I saw lots of damaged stereo's because a strap on the back was not used. I did not use a strap to install my Alpine in my 300D which has the same body as yours, W123.

Good Luck!
Tom

__________________
America: Land of the Free!

1977 300D: 300,000+ miles

American Honda: Factory Trained Technician & Honor Grad.
Formerly:
Shop Foreman;
Technical Advisor to Am. Honda;
Supervisor of Maintenance largest tree care co. in US for offices in Tex.