MX5 Diff Washer

[Johno]

Hi all,
This may be a dumb question but there is a large rubber stop washer under each side of the diff mounts on the donor car. I have checked through peoples builds and don't see any brackets on the chassis for refitting these. The question is
"are they to be left off and what are they for on the donor car"?

I guess Mazda put them there for a reason and I myself have fitted brackets in case I have to fit them but cannot find any new replacements as the original ones are a bit shabby to say the least.
Any ideas?