Today a bookplate of the 6th Duke of Portland was sold on eBay by lesIndavid.
The Arms are listed in my Bentinck armorial.
Arms: 1st & 4th Azure a cross moline Argent (for Bentinck) 2nd & 3rd Sable three stags’ heads cabossed Argent attired Or (for Cavendish).
Crests: 1st Out of a ducal coronet gold two arms counter embowed vested Gules on the hands gloves Or each holding an ostrich feather Argent (for Bentinck) 2nd A snake nowed proper (for Cavendish)
Supporters: Two lions double queued the dexter Or the sinister Sable.
Motto: Craignez Honte (Dread shame)