If it is a Siemens part it should have an E number. Put that into their website and they have exploded diagrams with all the parts and numbers. They will often sell you a replacement bit of you can find others with compatible parts based on the number.
Thanks for the nudge in the right direction. Found the part but unsurprisingly for quite an old microwave, no stock. Looks like a store in Russia may be the only source, I think I'll find a workaround!
If it is a Siemens part it should have an E number. Put that into their website and they have exploded diagrams with all the parts and numbers. They will often sell you a replacement bit of you can find others with compatible parts based on the number.