There were many types of custom foresight blade made, e.g. by Parker Hale, AJ Parker, Brindles, Stensbys, et al.

The problem is you are trying to track down a commodity which was being manufactured and retailed about 50+ years ago! All of the above-named gunsmiths have themselves closed up and disappeared.

Your options are to keep searching eBay, hoping that someone recognises what they are selling and lists it as such, or fabricate your own by adding material to an existing blade.