Yes, Charlotte, you are correct - there's a purled "seam stitch" down the back of those socks. Many of the Vintage socks have that. I am not quite sure why, other than perhaps it's a convenient beginning-of-the-round marker, and maybe it helped figure out the turning of the heel.

I leave it in even though it doesn't really seem to have a "modern" purpose, in this era of stitch markers and such. (Maybe it's kind of the sock version of an appendix*.)

The one drawback to the seam stitches - kind of like the seams in the old seamed stockings - is you have to adjust them to be sure they're straight so you don't look all sloppy. (No, I've never worn seamed stockings myself but have enough relatives of an age to have worn them to hear their praise of pantyhose over seamed stockings.)

(*Except, it may turn out that the appendix isn't so useless after all. I have to say I kind of like that potential explanation - as a refugium for bacteria when something comes in and really upsets the whole system so they can recolonize after the disease or whatever. Not sure what that means for people who've had to have theirs removed and their gut-flora recovery time following food poisoning or a round of antibiotics, but it's an interesting idea)