The little brazeon bit is a stop for the shift lever clamp. It is meant to prevent the clampon levers sliding downward on the downtube when the lever is pulled backward against the force of the rear derailleur spring. If the lever clamp is tight enough, the stop is not necessary, but it helps to save one from damaging the paint or downtube by over-tightening the clamp. On a PR10L the stop should be designed to match up with Simplex Criterium clampon shifters.
Regards,
Jerry Moos
Houston, TX
Dan, It is not necessary - I've had several nice bicycles made during the '60s and very early '70s. None had brazed-on shifter bosses and none had this little fitting. I was always under the impression that it was some sort of British thing but your Peugeot proves me wrong. In any even, if the shift lever mounting band is assembled properly (and checked ocassionally) you sill not need the fitting to keep the levers in place. Best regards, Fred Rednor - Arlington, Virginia