part.xml was in wrong folder, must be "admin/forms"

admin/forms/part.xml

@@ -0,0 +1,20 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<form>
+	<fieldset name="details" label="COM_JRON_STOCK_TAB_DETAILS">
+	<field
+		name="titel"
+		type="text"
+		label="JFIELD_NAME_LABEL"
+		description="JFIELD_NAME_DESC"
+		size="40"
+		required="true"
+	/>
+	<field
+		name="id"
+		type="text"
+		label="JGLOBAL_FIELD_ID_LABEL"
+		class="readonly"
+		default="0"
+		readonly="true"
+	/>
+</form>