from the pictures it is difficult to tell the difference between a porcelain dial and a normal dial? How long were these porcelain dial daytonas made? When were they made? Do they all have the Zenith movement? Does it state on original papers that the watch has a porcelain dial?

hallo boris

you will find this dials in the first zenith daytonas only.
the steel version porcelain (16520) you will find in the R series,
the gold version porcelain (16528) in the R series and in the early L series (L2...). they all have the mk1 bezel (until 200 units only)
you will not find any statement on the papers.

rgds
dieter