Given our lack of actual written "how to-s" from the period, my interpretation of these marks is that the original maker oversawed the baseline and then continued to saw down to release the inside corner of the waste. This makes waste removal nearly as simple as it is for a through dovetail.
I holdfast my pin board to the bench, face down, for sawing.
|Just a demo, so no protective block on the holdfast|
This allows me to saw the whole pin face by simply oversawing the baseline. To start, I simply tilt the toe of my saw up, and saw as deeply as I can to the end grain line. After that, I level the saw out and take shorter strokes with the toe of the saw to release all the waste down in the corner.
Do this on both sides, pick up your chisel (I prefer to use a narrow chisel as this assists in breaking out the grain rapidly with less force needed.
Once you have most of the waste chiseled on the inside face, put the board vertically in your face vise. Split down with a chisel to remove large chunks of the waste, leaving just a bit of wood to pare out down to your lines.
A couple of final paring cuts and you have a finished socket. Total time for this process in walnut is about 30 seconds (longer in this example because I had to keep stopping to take pictures). This is a fast and repeatable way to make these simple joints.
Bear in mind I'm a hard-core traditionalist and care very little for modern opinions and methods. Those saw cuts are evidence to me, not flaws. The men whose work I seek to understand did their jobs with speed and job-specific knowledge. The modern idea of "every surface is a show surface" is anachronistic to the period in which I have interest. The inside of a drawer front will never be seen by the end user and a true period craftsman would not lavish attention on such a surface when simply oversawing a short distance would speed the process.
You may wish to try this in your own work. Remember, though, the reason I do this is out of fidelity to the past. If you are doing modern work, I would caution you that many see this technique as a "shortcut" or as somehow less valid than spending 20 minutes chiseling with special-purpose tools to achieve a pristine inner surface. I have very little use for those people, but just keep this in mind.