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looks like Deutschland ~1965
greyhound transfer on downtube a helpful clue
several DE marques used this animal as part of their symbolism
one of the best known Victoria-Fahrrad-Werke:
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the german marque Fritz Windt employed a greyhound as its mascot in its emblems. here is a mudguard ornament from one of the firm's cycles:
in addition there is a german marque called simply Greyhound -
one helpful clue in attempting to determine the actual manufacturer of a german cycle is to look closely at the seat stay treatment
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Mathieu is a common germanic man's first name so searching on it is a bit of a challenge...
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fittings -
wheels not original. the Maillard quick release skewer in the rear hub did not launch until 1973. original wheels were likely steel wired-on rims with five-piece steel solid axle hubs from either WECO or Union.
chainset is Magistroni but bottom bracket fittings from another manufacturer. appears to be an oil bath set - perhaps from Thun.
pedals Union model U40.
frame's chain stay bridge ESGE plate style.
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