Simplicity 6621 vintage sewing pattern, dated 1960. 

Misses' One-Piece Dress:

Slim dress with long raglan sleeves gathered to buttoned cuffs, has back zipper and optional self tie belt. V. 1 gathers to yoke with lowered round neckline and loop and button back closing. V. 2 gathers to bias collar which turns to inside. 

Size  12:

Bust  32 inches / 81 cm

Waist 26 inches / 66 cm

Hip    34 inches / 86 cm

Pattern is UNPRINTED, checked complete. Envelope is in poor condition, has extensive tearing and yellowing.

More '60s patterns here:

http://allthepreciousthings.com/collections/1960s