Active or even reservist US service personnel requesting no mark = compliance.
As Ed says, theres a number of reasons they might ask for this least of which is the company commander telling them thats the only way they're going to be able to carry a non-mil spec knife. Seals, Rangers, MACV-SOG just to mention a few, all have carried sterile edged weapons on some missions.
Clandestine/unmarked knives go back to WW2 and perhaps beyond. They have great heritage in the military and, usually command high prices among military knife collectors.
A civilian wanting no mark = ?
I don't know why anyone would ask for that save one: in the event of finding a knife at a crime scene, a maker's mark would instantly lead to you and the subsequent questions as to who it was sold to in the first place.
Ed writes:
"If it's a civilian that wants the knife, they would have to give me a very good reason before I agreed not to mark the knife. "
Thats easy: they intend to harm someone with it. Perhaps an aggressor, perhaps not; harm none the less.
Troy writes:
"At the very least, I would add my mark under the scales, where it was hidden, but my work could be verified."
OK, so your secretly or overtly marked "poniard, dagger, boot knife, bowie knife" is found present at a crime scene. Now a really well paid lawyer includes YOU in the wrongful death/civil suit, he's bringing against the estate of the defendant. And of course, there is plenty of ammunition to show just how many of us make knives for the purpose of defense... or offense!
m