On the Bosch saws, a long thin screwdriver, cabinet style (no wings on the tip/tip same dimension as the shank), goes into the hole on the top, above the blade, and engages a slotted something that, when unscrewed, releases the blade.
I don't know about the pre-Bosch saws, but part of what made the Bosch saws famous in the early years was the so-called "T" shank that locked in better and broke less than the 1/4" setscrew-retained blades everyone else was using.
Interesting - this is very, very similar in general design to the early Bosch barrel-grip saws. I've owned one, with the same fixed foot (passed it along to my son).