My thoughts... Otto's signature looks legit on the target. The "38" stamp on the box label is likely a production code number or inspection number of some type, not an indication of caliber. The serial number should be hand written in grease pencil on the bottom of the box. However, if not in grease pencil, possibly a heavy black marker.
The photo of the label looks good and I would say authentic. The test target numbered to the gun, gives me confidence the box is correct (why keep the target and not the box kind of reasoning) but a photo of the top of the box and the serial number on the bottom would help.
If you are asking about the barrel length; its a 6".
If the serial number on the target matches the gun, the stocks are not original. In '68 it would have been fitted with "half moon" style checkered stocks. But if those are full checkered, you might prefer them.
Best of luck with this one. The gun looks really nice but the photos are not great.