Final answer:
In a machine sewn seam specification symbol, '301' represents the stitch type, particularly a lockstitch.
Step-by-step explanation:
In the machine sewn seam specification symbol 301-SSa-1, 301 represents the stitch type used in the seam construction. This is a part of the standardized system for seam specification used in the textile and garment manufacturing industries. The number 301 specifically denotes a lockstitch, which is created by two threads, the top thread and the bottom bobbin thread, interlocking in the middle of the fabric layers. The abbreviation SSa refers to the seam class and the superimposed seam type a, while the number 1 at the end denotes the number of rows of stitching.