Looks very interesting. I'd be concerned that the most likely place a German barrel might have come from would be a low pressure rimmed cartridge single shot. My other thought would be that if I was pretty sure sure of the provenance of the receiver/engraving, then I'd consider having a quality modern barrel machined in a close likeness to what you're trying to achieve.
A standard octagon to round barrel might be had for a pretty reasonable cost. An integral rib that looks good, might probably bump the anti way up. The right look from an orphan barrel might involve a good bit of luck. Is it worth rethinking the relatively large bore and high pressure based on serial number? Only thoughts, I'd love to stumble into something like that.
