MyE28.com

Preface, I don't know M21s. I don't even know M20s. But that doesn't look right.

It looks very similar to an M30 oil level sensor, which has an O ring, I'm pretty sure in the sensor, that seals to the pan. In this case you would need two, one on either side of the plastic piece. And Gadget as usual has an insight, one of those studs looks like it's cut off. But I just don't see them using a plastic shield to seal between the pan and the sensor.

https://www.realoem.com/bmw/enUS/showpa ... Id=11_0168

has the assembly diagram, they don't show nuts between the metal shield and the sensor, but just looking at the shape of the parts I just don't see how that would sit together right without something to keep the metal shield from bending. If it had spacer bushings or something in there it would make more sense to me.

it does appear that the studs unscrew though

Looks like the wrong sensor. Grind off the three tits so the plate will mount flush against the sensor.
Here is a picture from my engine that is currently being rebuilt with original sensor installed.
https://drive.google.com/file/d/13iSn1y ... share_link

Got it. Thanks

Just picked up an M21 and love seeing your build come along. I would be very interested in learning more and sourcing your rods. Do you have a company site or can I just PM you?