Regarding new frames of the classic era, I was under the impression that it was quite common to receive a new frame (and fork) directly from the paint shop. Turning a new frame into a complete bike required reaming, milling and tread chasing. For several reasons, some assemblers preferred doing this work themselves.
To free the assembler from the preparation work, especially the amateur without the proper tools, some frame builders offered "deluxe frame sets" as an option. The set included an installed head set, bottom bracket, and a seat pin. The additional cost for the "set" was often less that the price of the individual components. At least that's the way I remember it.
Pete Imandt Ramona, CA
.