Removing gearbox

[Brianmoooore]

Well, the reverse light switch is high up on the RH side, and I can't see it anywhere else in the pic., so perhaps that's what it is.
If the head has been knocked clean off, and the pin has dropped out, what's left will look like a hollow bolt, I suppose.
Reverse light switch is #19, of very poor internal design, has a very high failure rate, and I buy them in bulk. Certainly not NLA, and available from aftermarket sources as well as BMW. The switch from later models will also fit, but has a different connector.
The give away is that it has (had) two 1/4" male Lucar electrical connectors sticking out of it. Version from later models has a connector similar to the injectors, blue and brown temp. sensor, etc.

[Cloggy Saint]

The picture says #18 is the reversing light switch . The piece I broke was more or less as you described Brian, but it didn't have an electrical connector, it had a small valve. And no malc' it's not 3, 4 and 5, that's still in tact and attached to the box! I'll see if I can find the remains of it tomorrow and post a pic.

[daimlerman]

I'm with Brian,only thing it can be is reverse liight switch from your description,wrongly positioned in your diagram,19 is the correct position.Never seen a BMW reverse switch in 18's position,unless the autobox has it there,of course.

Both of my E30's have required replacement switches at some point,and other E30's that I have owned/played with have needed them as well.

[Brianmoooore]

Auto box doesn't have a reverse switch at all. It's combined with the inhibitor switch, and inside the car, fixed to the selector lever on these.
Just noticed that the diagram I posted shows 18 as the reverse light switch, and appears to go into the rear of the box. The only place I've ever seen a reverse light switch is on the right hand side of the box - #19.
What exactly did this box come from?

[Cloggy Saint]

It came from a 2.5 1990 manual cab'.