I can only point out from my point of view that serial numbers were only useful when exporting dozens of frames for the Customs and Shipping people.
Also a batch going to the painters would require a number of sorts to identify which frame had to be which colour. Sometimes they were in sequence and sometimes not, it bothers a builder not one bit if the frame is later sold on to another party other than the original customer..the customer of origin may or may not require a frame number for insurance purposes but that is really their concern and would be stamped on as requested,,,be it birth-date or S.S N or something akin to that.
I was interupted by my wife with a cup of tea ( or glass of Guiness, can't remember) whilst I was stamping a frame a few years ago...the interuption caused me to hit the number punch slightly off center and it shot off to Who Knows Where into the corner of the workshop....since then, no frame had number seven stamped on.
So it is highly probable that many "custom Built " frames are not stamped because since they are " one at a time"..don't need to be identified by the builder..A builder doesn't really care about whether the bike would be sold again along the line and honestly can't identify each and everyone by photo but may be able to when seen up close.
I just completed today a tandem frame for Margaret and I and stamped it with our birthdates...luckily no number seven was needed and hopefully this will be the last one and I can "off-load " all my building stuff.
COLIN LAING CHANDLER ARIZONA.USA