I was a tailor and it looks like silk top stitching thread. You would make the wool suit jacket and then top stitch with silk to make it look professional. Silk is usually very strong. You would not need a lot on a spool just to top stitch so it usually came on the smaller ones. If you find it breaks easily, don't use it. Instead, get an old green canning jar and put all the thread in. Set it on a shelf and enjoy the pretty colors of days-gone-by.