Ok there has been lots of confusion on this topic, but it looks like the E5-1620 V3, the older Haswell-E, chip is locked. So it is stuck at 3.5GHz. This is confusing as some folks have multiplier unlocked 1620s. Strange. This processors have 4 cores and 8 threads. The 1630 is also not supposed to be
But another poster says only the Ev-1650 V3 is unlocked (and the higher end 1660 and 1680). I just tried it and confirm the E5-1620 is not. So not as good as the various K chips that are out there (the i7-6700K) for instance, but it does have ECC.
Right now if you care about reliability and want to overclock, the Xeon E5-1650 V3 seems great. There is a rumor that the E5-1660 is also overclockable.
Note these are two generations older than the V4 and the V5 but the later versions are locked.
Pricing-wise the 1650 is six cores and $600, the 1660 is eight cores $1600!