Based on the model and serial number plate, your guitar is a 1960s Silvertone Model 1436, a vintage electric guitar manufactured in Japan by Teisco for Sears.
Model Details
Manufacturer: Produced by Teisco (Tokyo Electric Instrument and Sound Company) during the mid-1960s.
Branding: While manufactured by Teisco, these were branded as Silvertone for sale through Sears, Roebuck and Co..
Alternative Names: This model is virtually identical to the Teisco WG-2L (also known as the ET-220 or K-2L).
Key Features: It typically features a solid body, two single-coil pickups with rocker switches, and a distinctive "shark-fin" or "4+2" tuner headstock.
Identification & Dating
Production Era: These were primarily introduced around 1965 and retired in the late 60s.
Serial Number: The number 121399 on your plate is a production sequence number. While Japanese guitars from this era lack a centralized database for exact dating, the "1436" model designation is the primary identifier for its 1964–1966 production window.
Value & Collectibility
Vintage Silvertone 1436 models are considered "cult classics" among collectors for their unique "surf-era" aesthetics and "gold foil" style pickups. Depending on the condition and original parts (like the often-missing whammy bar), these typically sell for between $150 and $900 on the vintage market.