The WSTL18 uses an AVR microcontroller. Its in-system programming interface, whose six wires include the SPI, power lines, and reset. Those have all been broken out to a 6x1 1.27mm header for programming as well as availability of the SPI to any expansion shields.

Connecting single-core wires to the headers is ok for code development on a prototype, but not practical at all for (workshop) production programming, so I've designed an adaptor for the regular 2x3 ISP header.

On the board side, to the right, I placed both a 6x1 1.27mm header as well as long smd pads to solder pogo pins on, something like this but with all six pogos in a row.

On the AVR programmer connector, on the left, I've placed a traditional 2x3 0.1" connector as well as a 0.1" edge connector. This flexibility wasn't required but I've wanted to try these connections for a while, so this was my chance. I've added the eagle files for this on the project's Github ( go to Eagle -> WSTL18_ISP).