I shoot a number of antique firearms and transport them to the various international competitions. Customs don't have a problem with "No Maker's Name", "No Serial Number" on the certificate. Likewise, I've got three original Remington revolvers, one is on ticket and two have been "retired". Sussex FLO do not let me write a new antique acquisition onto my own certificate, it needs a visit to a dealer to do it (maybe his handwriting is more legible than mine?). Single shot pistols shouldn't be a problem in France, I think revolvers are dealt with differently. Anyone can write the new gun onto your certificate if they have possession at the time. If the seller has a shop stamp, that'd do...
The other post about serial numbers is good. All modern firearms have to have a serial number and customs will need to see it on the way in, it's no good saying, "it's under the woodwork", you'll be pulling it apart in the red channel!