My theory :
Weinmann originally offered 6-bump pads (36mm) pads. Then, with the introduction of centerpulls, they offered larger 7-bump pads (42mm). Then, for more surface area, they converted the 7-bump pads into 4-large-bump pads (42mm). To get more surface area on a 36mm pad, however, they could not offer a 3-large-bump pad (that would be roughly 31.5mm, or it would be inconsistent with the 4-bump pad.) So, they redesigned the 6-bump 36mm pads to be 'X' pads, and offered them in both red and black.
The new black jagwire pads, at a very compelling price for 50 pads / holders, are pretty faithful replicas of the 36-mm Weinmann X-pads. They have a 40mm holder and a 36mm surface area, and an 'X' shape to the pad surface :
They have excellent modulation and stopping power, in fact, I file down the edges and put them into my Campagnolo brake holders.
- Don Gillies
San Diego, CA, USA