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The fairy tale is real.

Legend has it that Intel prepared the Core i5-7660X three years ago, but never officially released the processor. A Chinese Twitter user recently obtained a working sample of the forgotten child from the Skylake-X family.

The Core i5-7660X clearly uses the Skylake microarchitecture. Therefore, the processor is manufactured on Intel's 14nm+ process node and slides into the LGA2066 socket just like any other Skylake-X chip. The Core i5-7660X inherited all of the attributes of Skylake-X, like support for quad-channel memory (128GB max) and AVX-512 instructions.

Had Intel released the Core i5-7660X, the hexa-core processor would have been an interesting offering at the time. The Core i5-7660X would not only be the chipmaker's first Core i5 HEDT (high-end desktop) processor, but also the first Core i5 SKU to support quad-channel memory.

Model Cores / Threads Base / Boost Clock (GHz) L3 Cache (MB) TDP (W)

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Core i9-7980XE 18 / 36 2.6 / 4.2 24.75 165
Core i9-7960X 16 / 32 2.8 / 4.2 22 165
Core i9-7940X 14 / 28 3.1 / 4.3 19.25 165
Core i9-7920X 12 / 24 2.9 / 4.3 16.5 140
Core i9-7900X 10 / 20 3.3 / 4.3 13.75 140
Core i7-7820X 8 / 16 3.6 / 4.3 11 140
Core i7-7800X 6 / 12 3.5 / 4.0 8.25 140
Core i5-7660X 6 / 6 3.4 / 5.0 8.25 140

The CPU-Z screenshots show the Core i5-7660X with six cores, six threads and 8.25MB of L3 cache. These parameters alone are enough to assume that the Core i5-7660X is closely related to the Core i7-7800X. 

The Core i5-7660X comes equipped with a 3.4 GHz base clock, which isn't a big deal. Its boost clock, however, is pretty impressive. Previous Skylake-X offerings maxed out at 4.3 GHz, and the Core i5-7660X exhibits a 5 GHz boost clock, the highest of all the Skylake-X SKUs.

At the end of the day, the Core i5-7660X is essentially a Core i7-7800X with higher clock speeds but without Hyper-Threading. It's rated with the same 140W TDP (thermal design power), but by disabling Hyper-Threading, Intel could jack the Core i5-7660X's boost clock through the roof.

As a matter of fact, the Core i5-7660X could have been a product of Intel recycling subpar silicon that didn't meet the standards for a Core i7-7800X. This would certainly explain the close similarity between the two HEDT processors.

Not that it matters anymore but, the Core i5-7660X provides 28 PCIe 3.0 lanes and supports DDR4-2400 memory modules just like the Core i7-7800X. As you might recall, only the Core i7-7820X and above natively supported DDR4-2666.

It remains a mystery why Intel never released the Core i5-7660X to the public. Probably Intel was concerned with either lack of interest, or it wanted to keep HEDT as a platform for Core i7 and above (never mind the short-lived i5-7640X). Word on the street is that some i5-7660X samples actually made it to the hands of a few selected overclockers.
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