I would go for a very slightly curved scale. The straight scales were on certain razors from that period, but I think the slight curvature in the scales suits the aesthetic of the blades with extremely curved spine (see first and third from the bottom on VoidMonster's last photo). Horn is advisable, if you can find a blond horn (any restorer can too). VoidMonster can also give some advice (he has tried many experiments) on using dyes to turn blonde horn into faux tortoise, like they did back then.