Simplicity 3123 vintage sewing pattern, dated 1959.

Teen age Basic Dress with Two Skirts, Detachable collars and A Belt:

Dress has round neckline, set in sleeves and wide self belt. Each view has detachable accessories. V. 1 has slim skirt, V. 2 has flared skirt. 

Size   12:

Bust   32 inches / 81 cm

Waist 25 inches / 64 cm

Hip    34 inches / 87 cm

Pattern is UNPRINTED, unused and factory folded. Envelope is in very good vintage condition.

More '50s patterns here:

https://allthepreciousthings.com/collections/1950s