Hi Wulffo ..Found it !....It says D7 but does not look like the usual type with string and float ; at least I can't see anywhere on the lower side for it to attach to a tube / pillar down to the tank .
Could it be if the type with float running up a spiral spindle (similar to Pfalz D types) with that bit missing .....Apart from 65l , no numbers..Strange ..
Does the D7 definately refer to " Fokker" .....A mystery , this one !

Regards John