Ref: undrilled flash holes- In the US, a muzzleloading firearm, regardless of age, is an antique, and falls outside of the 1968 Gun Control Act. As such, they can be shipped by US mail around the country, do not require Dealers to maintain paperwork, require no background check, etc.
That is not the law in ALL countries.
In some, a working Brown Bess is a firearm, and subject to a lot of legal restrictions. But until the flashole is drilled, it is simply a bit of metal tubing and a bit of wood traveling around together.
A goodly portion of what you are paying for is historical accuracy- of key importance to some re-enactors that will not wear underwear with machine stitching in it. Myself, I shoot a caplock Thompson-Center 50 cal Hawken rifle. It is not historically accurate- has a couple of coil springs, stainless nipple, micrometer sights etc. Would make a re-enactor swoon at the heresy- but it is damned reliable, and more accurate than I am.
You pays yer money, you takes your pick. But I would point out one thing- since there is no flashole, what you are buying has never been proof fired (and the company disavows resposibility for any gun altered to become a shooter. ) (translation- you're on your own)