From the deskthority wiki:
Vintage Type A: 80 cN, translucent white stem, black click-slider — 1987–?
Vintage Type B: 50 cN, translucent white stem, translucent white click-slider — 1989–?
Modern: 80 cN, white stem, black-slider — 2007–2012+?
The Type B has been reported to feel identical to modern Cherry MX Blue. Because of the harder types' similarity to Cherry MX Green, it has sometimes been nicknamed "albino green".
The sliders in the newer switches are proper white, while the older ones are translucent. The slider construction is a bit different. The spring is identical to the one in the vintage Type-A clicky switch and feels identical.
The switch is not to be confused with the Cherry MX Clear which also has translucent white sliders. Modern switches can be told apart more easily because their sliders are opaque, but vintage switches are visually almost indistinguishable to Clears.