Hi there,
after seeing many meticulously restored cars with horrible german look-alike plates, I'll try to outline the do's and don't.
I have not prepared for this, but I will add further Information by editing this post as I come along.
The information is not outright as there is nor rule without exception (Diplomats,Military,Dealer,temporary... ) but it should cover the majority of Cars between 64 and 73.
The plates within the 64-73 timeframe were all of the old "DIN-Schrift" font according to DIN 1451.
So they had no Blue thing on the left and the old font. They were available from 1st june 1956 (after introduction of the euro plates on 1st Jan 1994 upon request) until 31st Oct 2000. In some regions you are now again able to get them upon request if you have a historic car with the so-called H-Zulassung.
They have 1-3 Letters for the region code followed by "-" followed by 1-2 characters (usually no I and no O, but there are exceptions) and a number between 1 and 4 digits.
If they had two lines then the region code was on the upper line and the 1-2 letters followed by 1-4 digits on the second line with no "-" on the whole plate.
On the front and rear plate of the car there is either a Stamp (very oldschool) or a kind of Plastic chip "Feststoffplaketten" in an little pot or a sticker as a sign that the car is "zugelassen" it is called Dienstsiegel and is with regional differences (exeptions for Zoll and others...).
This stamp is always under the "-", exceptions to this are plates with two lines where it was placed left of the region code.
Euro Plates as of 1995 have no "-", between 1994 and 1995 they had a longer "-"
On the rear plate there is also the TÜV - Sticker or also as plastic chip in this little aluminium oder SS pot, placed above the "-" on plates with two lines they were placed right of the region code.
Here is a chart from wikipedia which shows which year had which color.
Earlyer plates had a shorter "-" than the later ones, and the "-" was vertically centered.
Larger lower stickers "Dienstsiegel" brought up the need to raise the "-" . But this was long after the time the Cars were built (I think late 80s).
Usually they were plain white and black with no reflective coating. Later the reflecting material became optional (perhaps until 91), and finally getting mandatory (if you had to get new plates).
Some regions as e.g. Landkreis LEOnberg (and Weissach belonging to it) had their own region code which changed during time.
Old Cars may keep their plates, but whenever there is a change of Ownership you will get new ones (BB as successor of LEO).
With the introduction of the ASU - Abgassonderuntersuchung (emission check) in 1985 there came also a second sticker (or plastic chip) on the front plate (being after 85 not of interest for this article).
Sticker / self adhesive plates as seen on many of the factory race cars were to get in the earlier days, nowadays only to be legal with a lot of trouble or you just make them and stick them on.
Maximum size is 520mm width and 110mm height. If you have a short number e.g. S-P 1 or S-P 11 you could have shorter (narrower) plates if you wanted to.
The DIN Font exists in 2 types middle and narrow. If you happen to have a car where a regular sized plate doesn't fit you got eg. narrow Font and a smaller width plate.
Narrow font should only be used for the 1 or two letters after the "-" with the regin code and the numbers in regular fontwidth.
For US Cars you could get plates with two lines and not only narrower font but also smaller font.
The surrounding line is 4,5 mm (Tolerance: +2,0 mm / -1,0 mm)
Zoll-Kennzeichen were for Cars that were to be exported: oval signs having a Z as first character on the second line.
An example of a TÜV Sticker post 74: these are with the month in which the next Check is scheduled on the top at 12 o'clock.
Some examples of don'ts
1.) Don't put new (euro) Font(94-) on old plates (-94):
The font for the plates changed with the introduction of the Euro-Plates (with the blue thing on the left)
do it rather like this one
Weblinks with further information
Photos of cars with factory plates
www.kennzeichenwelt.de
www.nummernschildmuseum.de/
Zollkennzeichen for export (thanks Dennis)
Confirmed Stuttgart plates with Date (not Factory plates, just Stuttgart or Leo plates where I have the date they were issued)
S-ML 96x July 61
S-E 92xx Apr. 68
LEO-MN 9x June 66