AISI steel grades:
http://en.wikipedia.org/wiki/AISI_steel_grades
Molybdenum Steels
40xx Mo 0.20% or 0.25%
44xx Mo 0.40% or 0.52%
http://en.wikipedia.org/wiki/Molybdenum
So it's steel with just Molybdenum
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Lemond Zurich, Cinelli Hobootleg Geo, ICan gravel bike, Tifosi Rostra, Specialized vado turbo