As far as I can tell, the left shoulder strap is made up of three layers of material. The outer layer is red velvet, the middle layer is canvas, and the inner layer is a white (tawed?) leather. The two outer (fabric) layers are a continuation of the breastplate covering. The inner layer of leather is sewn to the fabric layers around its exposed edges. I suspect that the edge of the leather that we can not see is caught up in the uppermost horizontal row of rivets at the top of the breastplate. The two edges of the shoulder strap that "show", that is the neck edge and the shoulder edge are bound in a strip of red velvet.
Between the fabric layers and the leather layer are three iron or steel strips. They are riveted through the velvet and canvas. They do not overlap one another or the breastplate. This is the surprising thing. I would have expected them to overlap. They are also shorter than the width of the shoulder strap. This presumable allows a bit more neck room in spite of the great width of the shoulder strap.
Here, where the velvet is partially torn away, we can see some traces of the canvas over the first plate.
I think we can presume that the leather is serving as a lining, rather than a load bearing layer. Thin, "glovey" leather like this would not contribute much strength, but it would keep the plates from abrading and catching on the wearer's doublet or his mail.
That shoulder straps were frequently plated inside can be inferred from the rivets and lines we see in art. They are pretty obvious here on Sir Ralph....
... but you have to sort of squint and imagine what they might have looked like when this fresco was in better condition.
Mac
