Sewbeadit has the "newer" version with the auto tension at the top. They are "automatic tension" as you really never adjust it. It's sort of a hard stop - it clamps the thread down tight and releases at a certain point to let thread flow, and then clamps down tight again. Sort of like an On/Off deal.
This is the older style Singer
This is the newer style (this one is set up as a treadle)
And for comparison, a W&G (later style)
and a National - made off the W&G design.
My frustration - while Boye made needles for the W&G machines, they did not make them for the Singer 24. I bought some off eBay.