Also apologies from myself littlebeetle.

Non-fixie, I am a little slow. I just realized that you are in Holland. "The Land of Cheese and Tulips", I thought maybe you were in Wisconsin.

I don't hear much about the Union brand name in western Canada.

There are bicycles here with "PARKER Process MIKI SAKAI Made in Japan" decals. I think, Miki built frames from Sakai, Osaka Prefecture, Japan. These decals are found on several brand names of bicycles.

So I would not be surprised if the Union brand name in Holland also had some frames from Miki. The serial numbers of your bikes are the same format as the Miki frames here.

Three brand names that had Miki Sakai frames:

Sekai from Seattle, Washington, USA (models 4000 and 5000)
Sakai, a house brand of Bloor Cycle, from Toronto, Ontario, Canada (models Star, Select, Ultra, and possibly more)
Rocky Mountain Bicycles from Vancouver, British Columbia, Canada (before the brand name was purchased by the Procycle Group) (model Sherpa in 1984, and possibly more)