http://www.c627627.com/AMD/Athlon64/
Looking at this chart the blue CPU's are Claws.
All the Claws before the 3500+ 512kb parts were all 2.4-2.6 1mb cache parts.
Seeing the data in the chart seems to make it clear that 3500+ cpus are
a. old stock steppings that did not sell as well as they should have
b. binned and/or failed 1mb cache parts that were cut back to 512kb cache
c. AMD made way to many of that stepping and need to make some money off of them. They are still getting $200+ each for them, not to bad if you have 20,000-30,000 parts to sell.
Personally I think they just made to many of that stepping, and they are making the best of it by making a part that sells better.