What's going on with the 1939 Register?

[Merry]

My relative who was born 6 Apr 1921 still doesn't show up, but I don't know whether that's because they haven't opened any records since the 100th anniversary of her birth, or because there is no record to open, or her dob is wrong on the Register or what.

You could see how many people are recorded as born 6 Apr in an earlier year and then compare it with the number showing born 6 Apr 1921 to determine whether they have opened those records yet (my guess is that they haven't!). You could do the same for other dates to make an informed guess at where they are up to with blanket openings of records.