Hi Len,
I double checked and that is indeed the diamond mark found on the back of the Chrysanthemum plate. I can also see what you mean as the Denmark plate has the same registry diamond! I have attached the full image of the back of the Chrysanthemum plate and Denmark plate (easy to tell as it has the pattern name) along with a close up of the Denmark plate. What a mystery! I'm not familiar with how specific pattern registry entries are but I'm assuming it would be uncommon for such different patterns to be deemed one style? Perhaps a mistake was made and this explains how one Chrysanthemum ended up with a set of Denmark patterns? I'm very much enjoying learning all about these things and very thankful for your insights!
Cheers,
Anna