Where do you find these letters at?
And Are these casing numbers or stamps, by the builder?
If this is an “S” stamped between the cylinder and the head, then it was stamped by the builder referring to a standard piston size and cylinder jug.
The “SEC” part, I have no idea
The only other important casing numbers to look for are on the right side of the motor, above the bottom end, and at the bottom of the cylinder ( 787 )
I hope this helps, rather than creating more confusion, just my 2 cents. Fast Freddy.