According to an Ideal Reloading Manual, (1890's) reprint, Wolfe Publishing 1991, the rearmost Mould List shows, "512138.(450gr.)
This bullet was first made by our Mr. Barlow for a heavier ball for the .50-95 Winchester. It was afterwards adopted as the standard bullet for the .50-110-450 (.50-100-450, actually) Winchester." (I'm assuming that this is the Mould being used.)
I believe that A.C.Gould MAY have designed this bullet, and had the Ideal Co. cut the Mould for him, but those books are packed, and can't verify. My immediate assumption was that the bullet was designed for the Winchester Single-Shot (Hi-Wall) chambered for the same Ctg. My belief was that this longer bullet, if seated into the case SO AS TO CYCLE PROPERLY IN THE WIN. 1876, the bottom most lubricated grooves might protrude past the neck, into the powder, and cause powder contamination, or aggravate barrel fouling, whereas in the "Hi-Wall", one MIGHT be able to seat the bullet out enough to eliminate the possibility. I currently don't have an Ideal/Lyman #512138, so that I can't check this myself. Please do post with details, and let us know how this loading works for you, as I have wondered myself! If your friend has a different Mould, please post the Manufacturer/Number!
I will be sharing the words in your post with him.