I got a used 501 wheelset cheap, and i think the hubs are 1st-generation ones, with MAVIC stamped into the hub barrels. The axles are 100% aluminum, and they can get bent. I don't think mine are bent because a local san diego "mavic expert" tested it (which consists of rolling the spindle on a flat table.)
I see a lot of singleton rear hubs sold on ebay, i think there is demand for them. axle sets are available from sjs cycles. I don't think they break; i think they get bent. i myself am looking for a 32H full hub : my rear hub has a tick, noticeable during acceleration, and visits to the arguably #1 and #2 shops in san diego have not solved the ticking problem, although the #2 shop's mavic master mechanic glued in new bearings (the 6th and 7th bearings that have been installed on this stupid hub trying to determine the problem), and the problem disappeared for a few months, only to reappear.
This experience has taught me to HATE cartridge-bearing hubs.
- Don Gillies
San Diego, CA