Hmph! That does look like a "6,81"...The first & second number stamps are different...However, my 7.65 Luger commercial barrel measures 7.60mm land-to-land, not 6.81mm...But "8,81" would be a 9mm barrel...
It could have been sleeved...Lots of basement gunhacks do sleeving...
Bruce, you don't have to hide the serial number...the original owner died long ago and his government is not likely to want it back...
