I am interested in the use of targe type shields by the Scots and other Gaelic people. In Ancient Scottish Weapons, Joseph Anderson cites a law regarding their construction, stating "The use of the target in Scotland was not confined to the Highlands. The statutory equipment appointed by the Act of 1425, for such yeomen or burgesses as were not archers, was “sword and buckler, and a good axe or broggit staff;” and in 1481 the axemen who had neither spear nor bow were required to provide themselves with targes “of tree “or leather, according to patterns which were sent to each of the sheriffs. The watchers of the burgh of Peebles, in 1569, were armed with jack and spear, sword and buckler." I take "of tree" to be a reference to the cross lain, doweled boards that would ordinarily be covered with leather. Does the "or leather" refer to a leather skin? Or is there any evidence of a targe made entirely of leather? It seems that would be of limited use, unless the object of a leather targe is to catch a thrust.

Those targes still in existence were made of wood and covered with leather, not rawhide. I think the reference you cited is referring to shields made of wood only, i.e. without a leather cover, versus those of wood with a leather cover.

Out of curiosity, I had bought a full skin of dry bull rawhide, which is about 4mm thick or so. You have no idea how tough that is. I had to cut it with snips, and I would compare it with 2mm thick steel for the force required to cut it, but unlike steel it stays straight after the cut (metal tends to curl). It is very stiff and doesn't tear. It's actually quite comparable to a modern, impact-resistant plastic like polypropylene, only much stiffer for the weight.
The trouble is that it's very responsive to humidity. Not just a few drops of water, but it has to be stored in a dry place. For the moment I still have most of the skin rolled, but I'm pretty sure if I was to shape it as a shield (which would be quite a tedious process), the finished shield would often deform and warp as humidity in the atmosphere changes. Maybe it could be oiled and waxed to prevent this, though.

Semantics are important when discussing technical issues. The targe was a pretty large shield. Froissart described a wounded knight being carried off the field on one so they were fairly substantial. The target was the smaller buckler-type version. The word target literally means "little targe".

The way I see it, we shouldn't imagine that the word targe applied to any particular type of shield. Generally speaking the words targe and shield were synonyms and could be used interchangeably. So when we see the word targe in 15th century legislation we shouldn't take this as strong evidence for the Highland style targe at this time.

The description given for these targes suggests that they could be made from wood OR leather, not both. Of course this doesn't match what we know of Highland targes. As far as I'm concerned these targes mentioned in legislation have nothing to do with Highland targes, they are interesting in their own right though.