You will also want the German manual "H. Dv 481/2 Merkblatt fur die Munition der 2 cm Waffen". It might be available in the download section of this site.
Try to get these books and you will have everything you need to know about the 20x138B cartridge.
"Die Patrone 20 x 138 B, ihre Verwendung in der 2-cm-Flak 30 und 2-cm-Flak 38 sowie in anderen Waffen." Vol. 1 and Vol.2
Jariber,
Flak 38 and Madsen (Sprenggranate and Brandsprenggranate) in comparisonView attachment 156515