These sights seem to be fairly scarce and probably were probably not especially useful most of the time so probably were lost or abandonned. The ones that I have seen offered are in relic condition, which don't much interest me, such as the rusty one pictured in the thread.
However, there is an accessory in the pic that is exceedingly obscure, and never mentioned, and is probably a field expedient, despite appearing quite well made. The elbow rests bolted to the rear of the trail are unique and have not been identified anywhere that I know of. Anyone who has laid down behind a 1910 and spent some time firing from the prone position, quickly discovers how uncomfortable and inefficient it is to do and the elbow rests are an excellent addition.