Preparing the lower sole for sewing: This time I chose to go with the method found with many originals, that is using a tunnel stitch. After cutting out the lower sole I prepared it for stitching by making the holes. This works best with a bent awl, as available from leather working supply shops. As you can see I first tried it out somewhere in the middle. Before doing so, I soaked the sole in warm water for a couple hours to soften it, as the sole leather I am using (4-5 mm thick, most originals range from 3-5 mm) is very, very hard.

I managed to only tear out one of those holes, pretty good for a first try. Another method that I have practised until now and somewhat easier, is to cut in the sole from the side to a depth of about 10 mm and then sew through the upper of the 2 resulting sections. This has not been found with any originals, though, AFAIK. Here's a picture what this looks like after sewing:

Sewing the middle to the lower sole: I again used one of those flexible steel needles for that, as explained with the back seam above. Probably a real pig's bristle would even work better as flexibility is the most important thing for getting through those curved holes

Here's a look from the side, before pulling tight the yarn, compared to a sketch from Göpfrich's Lederfunde aus Mainz:

I decided to cover the inside of the backseam as well with a strip of leather to prevent chafing of the heel. At the same time I used it fix the overstanding bits of the white leather strips I used for the outside protective leather band. First I drew the upper ends through the overstandin piece of the outside protective leather band

then covered it with the inside leather band and sew it all fast.

On with the last few steps: Before nailing the insole needs to be glued in. I am simply using modern glue here. The main function is to hold the insole right in place during nailing. I decided to try and do what was mostly done with the original caligae, i.e. nailing through all 3 layers, outer, middle and insole. The insole naturally has to be a little bit narrower and shorter than the outer sole. I made a cardboard duplicate of the outer sole and reduced it in size until it was right and I could use it as a pattern for cutting out the leather insoles. For this I'm using purpose tanned insole leather (according to the tanner I'm buying from, don't ask me for any details on this, I'm simply repeating his opinion here).