Don't know if this will help. But my 1981 W123 300D trunk
would not open and I thought that the key was not a master
(thought it was a Valet key). It is a brass key (that is bad).

It ended up that when I put the key in the lock, it was sort of
going in too far. I had to bring it back out just a little (like 1/32").
And carefully turn left to bring the button counter clockwise to push in and open.

It was really tricky at first. Just my 2 cents.

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80 300D 340K Owned 30 yrs
83 300SD 440K Owned 9 yrs - Daily Driver 150mi/day
02 Z71 Suburban 117,000
15 Toyota Prius 2600 miles
00 Harley Sportster 24k
09 Yamaha R6
03 Ninja 250