To use a saying coined on your side of the "pond", I think that Ted Baer asked if not the $64000 question then certainly the $32000 one, and I fear that he will never receive a fully objective explanation from any framebuilder simply because there isn't one. It's a very subjective topic. I think that that renowned Chinese rickshaw framebuilder to the Mandarins of the second Ming Dynasty, Confucius, summed it up as well as anyone has. Confucious he said " .. as many as there framebuilders...so there will be just as many opinions on the good finishing of lugs" It could be maintained that such a process is merely cosmetic, entirely unnecessary, and adds little if anything to a frame except for artistic titivation.. As Brian Bayliss pointed out, the "good finishing" should in fact be "good preparation" ie cleaning and profililing before the frame is brazed up. If done properly at that stage, there would only be the light stropping with emery, or preferably aluminium oxide cloth, to carry out on completion of the building process. I'm not sure at what stage in the history of framebuilding the "post-brazing" finishing process started. I recall that in the 50s when we built frames with intricate curly lugs and windows we would "flux up" the joints and leave them over-night to dry. On the morrow we would very carefully with an artist's fine brush, brush away any surplus dry flux from around the joint areas, in an attempt to avoid getting any unwanted braze material on the joint. The brazing in those days was carried out with coal gas/ compressed air torches that provided a large enveloping flame. This procedure was used because in those days too, it was difficult to find companies to sandblast or bead-blast frames, and many small builders could not afford to install their own blasting cabinets. More often than not the lugs would need a cleaning up with a very fine file and then emery cloth. Prodding and poking about with jewellers' needle files was not unknown either.As for gas-fluxers.. well wish on.
I recall that we used to use cast lugs which I think were cast in sand moulds. The lugs inevitably would be on the thick side and often still bore casting "flash" on them as well as seams. This meant that the lugs were filed to provide accurate profiles and filed to thin the wall thickness down, so that when they were assembled with the tubes and heated up with the coal gas torch, it would not take as long as if the lugs were still thick. remember that in those days too, some of the gauges of Reynolds 531 were pretty fine. In order to make the lugs look more elegant and with the dual purpose of not filing away the strength of the lug, the lugs would be tapered or "feathered" towards the edges during the preparation stage. Prolonged heating of the joints was avoided.
I think that the practice of filing lugs down after brazing came in with pressed lugs, in order to obtain the purposeful and intentional feathering effect observed on earlier cast-lugged frames. It became more of a cosmetic exercise than a functional one. It has to be added that on some of the later long point "windowless" lugs the effect of feathering the lugs was very attractive, but it would have been more useful, in reducing the heat input required, to have filed them down beforehand.
I still meet up with "old codgers" riding round the lanes where I live who are still riding the Rotrax or Gillott that they had been given almost 50 years ago as a reward for passing their school exams. They still talk about this intangible quality of their frames - when compared with far newer mounts - of " get up and go" To me I suppose it's meant to convey a sense of liveliness and response as opposed to deadness. I can't account for the amount of "get up and go" in any of these vintage frames except that I feel, cant really express it differently from that.. I don't know for certain.. that it's down to the torches and the lower input of heat of those days.
When intricate or elaborate lugs were cut, or fretted, by hand, it was only a matter of course that the edges of the cuts were cleaned smooth and square. Later pressed lugs did need a considerable amount of "cleaning" up to make them attractive. An additional reason for pre-brazing cleaning and profiling of the lugs thereby removing rough edges is to provide a smooth adge that facilitates the flow of the brazing material around the outline of the lug.
Norris Lockley... wishing that I too had more .."get up and go". Settle UK