Trust me. I worked at a Peugeot shop in the early 1970s. I have owned two UO-8s from that period, one of which I just rode to work this morning. Your decal set and components scream "early 1970s" to me. Yes, it was very common for enlightened owners and bike shops to stamp their own numbers in the frame, and the left rear dropout was one of the best sites for this. Bike theft was rampant during the bike boom years, and people rightfully got paranoid about those stupid riveted-on serial number tags. (My current UO-8 has both the original tag and my wife's social security number, which I stamped into the right rear dropout when I custom-built the bike for her from the bare frame).
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"Far and away the best prize that life offers is the chance to work hard at work worth doing." --Theodore Roosevelt
Capo: 1959 Modell Campagnolo, S/N 40324; 1960 Sieger (2), S/N 42624, 42597
Carlton: 1962 Franco Suisse, S/N K7911
Peugeot: 1970 UO-8, S/N 0010468
Bianchi: 1982 Campione d'Italia, S/N 1.M9914
Schwinn: 1988 Project KOM-10, S/N F804069