In addition to the usual scepter over K.P.M mark, most of KPM plaques had various numbers impressed on their back.
The purpose of those numbers was to indicate the size of the porcelain plaque.

KPM produced oval plaques in several standard sizes. Each standard size had a numeric single-digit code associated with it.
This numeric code was impressed right under letters "K.P.M".
Here are some examples:

2 - 5 ½" by 4"
3 - 7 ½" by 5 ½"
4 - 9" by 6"
5 - 11" by 8 ½"
6 - 13 ½" by 11 ½"
0 - 17" x 14"
I believe that some
1 plaques were made as well. They were as small as 3 ½" by 2 ½". I have never seen one though.
KPM always specialized in larger more expensive porcelain painting leaving smaller cheaper plaque production to

Unlike ovals, rectangular KPM plaques came in all kinds of sizes. No size codes were used. Instead, the actual length and width
were eiither incised in inches or impressed in milimeters on the back of the plaque. Older plaques are usually marked in inches,
while the ones made circa and post 1900 are marked in centimeters.   

It's a common mistake to assume that a plaque which does not have any numbers on its back is not a true KPM.  While the vast
majority of KPMs do have the numbers, there are many that do not, especially the oldest ones.  
Categories  >  KPM Plaques  >  Marks and Sizes