So the rubber part would be on the engine itself, right?
Looks more like the rubber protection from where the wiring harness goes through the metal plate to the fuse box on an early Beta, it has that horseshoe shape - it certainly looks big enough, either that or you have very small hands.
Grommets around HT leads are smaller.
I hope these two links are allowed?
https://oldtimer-shop.eu/sites/default/files/styles/product_velika/public/product-pictures/4581-4_3.jpg?itok=M-lYDVRtand
https://www.ebay.co.uk/itm/Lancia-Beta-Coupe-serie-1-fusebox-relays-cluster/303568395238?hash=item46ae161be6:g:S98AAOSwbNtevpR4Check to see if its still where it should be but don't worry if is fitted as it could have been dropped and another has been fitted. Having stripped many Fiats and Lancias over the years you find new clips, grommets, even pedal rubbers discarded and dropped by the people who assemble the cars.