Hahaha, two copies for sure! I've read the few sewing machine history books I could get my hands on, but find them frustratingly incomplete.
I've never actually seen the older 15s with the rotary hook, I've only read about them and have seen a photo or two. As the needle is always threaded in the direction of the hook, assuming the needle falls to the right of the hook as it does with the other 15s, I'd suppose it threads right to left.
I'm really curious as to why the model 15s ditched the rotary hook in favor of the oscillating; I'd have assumed the former would be smoother than the latter. Then again, I suppose Singer knew what they were doing....