Hi Grundy
Rick is absolutely right. The terms 'Union' and 'Registration District' were often used pretty much interchangeably, because when Civil Registration was set up in 1837 it used the Poor Law Union boundaries that had been established only 3 years earlier.
The post of Superintendent Registrar was initially offered to the Clerk to the Board of Guardians of the Union, and many of the registrars of births, deaths and marriages were also Union officers so the two systems were intertwined for a long time.
There were a few cases where very small unions were amalagated with larger adjacent ones, but in the vast majority of cases the Union and the RD were identical.
Mean_genie