These are duraluminum manufacturer id stamps.
I had to study the subject a while ago to review a modeling product. Note the pic of a Me-262 with unpainted panels showing these stencils:
MDC - German WWII duralumin manufacturers stamps decals (1/32 scale)
HTH
Rato Marczak
Danny,
I got the same manual from eflightmanuals. After writing to them about the lack of drawings, they told me that the drawings will be scanned as a separated manual, but they don't know when as yet.
Rato