30m QRSS/WSPR crystal combi

Short notice about a combination a accidentally saw:

• 18.138 - 8.000 = 10.138

The beauty here, it is subtractive, thus, thermal effects should (nearly) cancel out.
The 18.138 crystal is made available by VK1AA.

In terms of design, this [click] looks like an interesting approach. Although, I consider the use of 74HC4066s.
Thinking of a design, a 4MHz subharmonic l.o. comes to my mind, after all, subharmonic mixers are also bi-directional.