I really thank you, guys!
Just received the rifle this morning and Bill was right: it appears to be a rebarrel job, not only because there are no company markings on the barrel and no proof marks, but also because the gunsmith made an obvious mistake when he stamped the caliber, forgetting an "A" and correcting it afterwards.
The gun appears to never have been fired and I had some concern when I thought to notice a very fine hairline crack near the breach (gasp!) that turned out to be a long shallow scratch probably caused by some rough handling in the gunstore warehouse. Otherwise, the barrel bore is perfect, so is the crown.
I do not have cartridges yet, but I checked the headspace with a brand new Norma .22-250 case and a piece of masking tape between the bolt face and the case head.
The bolt close with difficulty on a .0075" thick "gauge". So, resized and "false shouldered" .22-250 cases could be used as an expedient.
In my neck of the woods (Brittany or "Bretagne" in Frog, a region that was stolen five centuries ago by the King of France) the .257 caliber is almost unknown, as I discovered when I tried to find ammo for the Ruger #1 Varmint in .25-06 that I brought with me when I came back home after a very long stint in Greenville, SC.
Only Speer flat base hunting bullets were available and I’m afraid the situation didn't improve. My plan B would be to resize .264 bullets with a Lee die and check the accuracy I can obtain.
Thanks for your attention.