As I understand it the mending involved splicing old and new threads together. The testing was presumably on both threads that were wound around each other and so giving an in between date with more of the thread being from the replaced thread origin.
The story of the ‘patch’ hypothesis is an interesting tale of bold presentation followed by cautious retraction. There are only a few authenticists who still believe in it all.
First, Sue Bedford and Joe Marino drew a line down the middle of the radiocarbon sample (along one of the ‘spines’ of the herringbone weave, and announced that one side was 16th century and the other side was 1st century. They assumed that each tested subsample consisted of a bit of both sides, so that the combination produced the 14th century date. Two things showed that this was wrong - firstly that there was no consideration of how the two different dates of fabric were fitted together, and secondly that the samples were actually cut up is various ways, so that some should have been 1st century, some 16th century, and only the ones containing both ages could have been 14th century. This was not what was observed.
So the ‘patch’ hypothesis was amended to become the ‘invisible mending’ hypothesis, whereby it was proposed that over the entire area of the radiocarbon sample there were a few (about 35%) original threads, and that new threads (65%) had been woven in among them, according to a skilled and almost forgotten technique called “invisible mending”. Textile restoration expert Mechthild Flury-Lemberg said that such restoration, of which she had personal experience, was visible on close observation on one side, and much more easily visible on the back. Joe and Sue found a company called Without a Trace, who, they said, claimed that by using “invisible French weaving” they could produce a mend which was invisible on both sides. Actually they didn’t claim that, and when I paid for them to produce a sample of their invisible mending it conformed precisely to Flury-Lemberg’s observations.
Undeterred, the stout defenders have retreated to the “invisible splicing” hypothesis. In this, broken threads are unravelled by a few millimetres, and new thread, also unravelled by a few millimetres, is twisted among it to extend it as far as necessary, and all the threads so formed rewoven back together to replicate the Shroud weave. Over the area of the radiocarbon and Raes samples, this would involve the individual splicing of about 400 threads, with such skill that not one ‘splice’ was detectable, all within an area of a few square centimetres. I do not believe this is possible, I do not believe it has even been done, and I have challenged those who support this hypothesis to show that the hypothesis is at all credible.