B3 while a commonly repeated problem, and oddly never proven with pics, can cause no reverse of course. But there is also a bulletin posted about a broken actuator spring or seals that can cause the issue. It would be the first thing I would look at. $8 part, or about, and much easier for most to work on.

MERCEDES 722.3 - 722.4 SLIPPING AND/OR NO REVERSE
COMPLAINT: Before and/or after rebuild, any Mercedes equipped with the 722.3 - 722.4 transmission may exhibit a chattering in reverse, slipping in reverse, or no reverse.

CAUSE: The cause may be, a stuck or sticking Accumulator Switching On Control Valve, a collapsed or broken Accumulator Switching On Control Valve Spring, a missing, cut, or leaking seal ring on the accumulator piston, or a cracked Accumulator Switching On Piston (See Figure 1).

CORRECTION: Remove and disassemble the valve body. Inspect and replace as necessary, the parts and concerns listed above, as shown in Figure 1.

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car