I might have them do two molds, one an exact copy of that schematic and one with the measurements shifted to get a .660 Nessler, like the one named in those French trials with the Nesslers, they list a Nessler in a mm caliber that converted to .660.
That way I'll have both. I have 3 .69 muskets and I'll probably have more in the future, might as well have fun with them.