20 x 138 question

What does the white color signify?
What year did Germany start making the 20 x 138
I have a number of rounds from 1939 to 1944.

P 31 20 x 138 a P 31 20 x 138

1 Like

This is a TP-T for export to Sweden.
The first cases made were not dated if I remember correctly and should go back to 1930 or even earlier.