AMD Athlon 64 3500+

Just like this Athlon 64 3500+ but with a different production date.

Both have the Venice E6 core. As time passed by (since the first Athlon 64 / Opteron engineering sample) AMD was tuning the existing cores to make small improvements (better on-die memory controller, scalability, bugfixes, etc.) which means there are many different CPU's that carry the number '3500+' You can find the 3500+ on S939 with Clawhammer CG, Newcastle CG, Winchester D0, San Diege E4, Venice E3 and E6 and Manchester E4. The latter is actually a dual-core chip but with one core disabled so effectively it's a single-core .

Disabling parts on a chip is something that happens quite often. The performance products have more cores, L2-cache or something similar. In case one core or a part of the L2-cache is defective the chip has to be rejected. To avoid this chip manufacturers (not only AMD but also ATi, NVIDIA and Intel for example) disable parts of the chip and sell it for a lower price.