WMF Warming Plate, Water Bath; Blue Onion? Age?

I think I've paraded this before y'all, perhaps in Metalware...but cannot find confirmation by Search...an' I've such a poor memory that there's nothing left of your instructive comments, if so.
It's ~9 3/8" at the rim an' ~1 7/8" high. Weight is ~30 oz, bone dry.
"Blue Onion" sticks in my alleged mind...but whether that is correctly associated with this pattern is uncertain at best, to me.
It differs from the ones I recall from my earliest days; my brother an' I each had a vessel of similar purpose, hot water used to keep our grub reasonably warm until we finished eating, but they were divided into three compartments to keep us from easily mixing our mush with our goombah an' glop. Also I cannot recall either of them having a metal base; I think they were all ceramic, with cork stoppers...an' were probably not of European make.
What are they properly called...an is Blue Onion accurate?? Any other pertinent thoughts are most welcome...Thanks for lookin'!

 

Here's one for sale
https://www.ebay.com/itm/265093937021?gQT=1

and another
https://www.etsy.com/listing/1102318441/16-wmf-blue-onion-warming-platter?gpla=1&gao=1&&gQT=1

 

OK...1920's, an' it is Blue Onion. I'm reasonably gratified...now to see if eBay has sold any of them recently.
Is the term 'white metal' generic for...what? Non-ferrous; CuNi...or closer to spelter?
Any ideas about the '51' above GERMANY?
Thank you, INH!