The problem here is that there probably isn't a suitable plate. Having looked through all the scans I have of the promotional cards, there are
Golden, Ruby, Pearl and Silver (text used, so not suitable)Golden Ruby and Silver - no text, but little room for numbersAs above, from the 'Jewel' range, but numbers presentA large generic 'Wedding Anniversary' oval plate with wavy edge and floral decoration (a yellow rose) where numbers could be added I'm not saying this is the definitive list, but that's all the information I have.
35th is Coral, so I suppose a Silver or Pearl could be adapted, if there was room for the numbers. 45th is Sapphire, but no such plate exists. I suppose Blue Greco (quite scarce) or similar could be used, but that's assuming there was room for numbers.
But what do you use for numbers? Stick-on ones would not look good.
Letraset type would easily rub off, but are there gold or silver colours?