In Answer to your question when these bikes were produced they would have been fitted with open bearings DDR and a sort of a rubber washer would have been fitted between that plate and the bearing to protect from ingress and dirt ETC. I had a couple of wheels built last year and read on a couple of forums that closed bearings are a modern choice and do not require any protection. Other advise may differ but the closed bearings work for me. I have even knocked out the open type and replaced with the closed type… I am a fair weather rider to be honest!!!
You can buy the closed type ( I use bearing options on EBAY) the manufacturer is Challenge and the number is 6302 ZC3 ( guess the ZC3 denotes closed type)
Best of luck Stuart