Conceived in late 1955, the Musicmaster (one pickup) and Duo-Sonic (two pickups) were Fender's entry-level student model guitars. The first Musicmaster appeared in spring 1956, with the Duo-Sonic making its debut a few months later. Both guitars featured a 22.5" scale length and an understated double-cutaway slab body. Like many other Fender models from this time period, they are distinguished by their one-piece maple necks and gold anodized aluminum pickguards. The lone finish offered during this period was called Desert Sand.
ADDED 4/05: front and back scans of repro 1957 Fender dealer information sheets for the Duo-Sonic (also shown on pages 232-233 of Fender: The Inside Story by Forrest White)

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• Case study: one of the very first Musicmasters (SN 10769)
  The most noticeable difference between the first production run of MMs and the standard '56-'59 production model was the black painted sheet metal pickguards. This guitar also featured an ash body (with routs connected by hand with a chisel), a neck date of 4/56, and pots dating from late '55.
  Restored by Jim Shine
  
  
• Musicmaster (SN 09681)
  Another of the very first batch of Musicmasters, apparently all-original except for the tuning machine buttons and perhaps the pickup cover. Given its scary-mint condition, I can't believe they only asked $885 for this guitar.
  Sold by Elderly Instruments
  
  
• Super-clean Musicmaster: front | back
  Source: T. Pershing
  
  
• Case study: Musicmaster, with original red-lined case (SN 1XXXX):
  front | back | body, front | body, back
  headstock, front | headstock, back
  Source: unknown (eBay)
  
  
• Duo-Sonic
  From GGJaguar's Guitarium and Ampeteria
  
  

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• Nice Duo-Sonic, owned by Ed Kelly
  
  
• Duo-Sonic
  Source: ??? (personally scanned from a lone page of an unknown publication)
  
  
• Case study: clean Duo-Sonic, with original red-lined case (SN 14895):
  headstock (front and back) | neck | fingerboard, detail -- nice "birdseye"!
  front | back | body, front | body routs
  pickguard assembly, front | pickguard assembly, back | electronics, side
  lying in case | "case candy," with original tuning machines (sans buttons)
  Source: unknown (eBay)
  
  
• Musicmaster: body | headstock | in original (red-lined) case
  Source: unknown (eBay)
  
  
• Musicmaster
  Source: Cosmo Gakki (Japan)
  
  
• Clean Musicmaster: front | body (both large images)
  Source: unknown (eBay)
  
  

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• Duo-Sonic
  Owned by Genji Nakano/Genzo Vintage Guitars
  
  
• Case study: nearly immaculate Duo-Sonic:
  in (red-lined) case | front of body | back of body | headstock | front of neck
  Source: unknown (eBay)
  
  
• Case study: clean, "closet classic"-esque Duo-Sonic:
  front | back | body, front | body, back
  headstock | original case
  Source: unknown (eBay)
  
  
• Immaculate Musicmaster
  With period-correct Fender Champ and Fender dealer stand
  Owned by Terry Foster
  
  
• Musicmaster, lying on original case. Clean!
  Source: unknown (eBay)
  
  
• Musicmaster with '59 Champ
  With original strap, cord, and case; everything you need!
  Source: unknown (either eBay, or Music Man)
  
  
• Musicmaster (SN 025910):
  front, in case | front | headstock
  Owned by Doug Moore
  
  
• Case study: Musicmaster (SN 027698):
  front (partial) | back | body, back
  back of pickguard assembly | electronics (note sausage-shaped tone cap!)
  headstock, back | headstock, front | neck date (5-58)
  Source: unknown (eBay)
  
  
• Case study: Musicmaster, with original case (SN 28991):
  front | back | body, front | headstock, front
  Source: unknown (eBay)
  
  

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• Musicmaster: front | back
  Source: Chris' Vintage Guitars
  
  
• Musicmaster: closeup of front of body | headstock
  Non-original pickup cover, but nice!
  Source: unknown (eBay)
  
  
• Musicmaster, very clean.
  Source: Guitar Center
  
  
• Musicmaster, lying in original (orange-lined) case.
  Source: unknown (eBay)
  
  
• Musicmaster
  "Born in February 1959," owned by Mike Robinson
  
  
  

  
  One of Robert Perine's great "won't part" ads from 1959,
  starring Larry Collins of the Collins Kids (regulars on "Town Hall Party").
  source: eBay seller plino