Hyvää päivää maaseudun Itä Englannista!
I'm sorry to say that the Lyman bullet is the best there is for this rifle - it is the bullet that we all use in the Muzzle Loading Association of Great Britain - MLAGB - and also in the long-range BP rifle shooting association of USA and Canada, Australia, Germany and so on. We shoot this bullet out to 1200 yards/1000m with great success.
The bullet you showed me is the actual one that I use in my Winchester High-Wall - it IS the correct diameter for a .45-70 calibre CARTRIDGE-firing rifle but the pointy-nose design is NOT suitable for a muzzle-loader like yours. Sorry.
If you can't get the mould from your guy - look up the old English aphorism/saying 'dog in a manger' - then let me know and I'll put you in touch with my pal Joe in Oregon. We've been dealing BP stuff for many years and his prices are the best I've ever seen.
Best graders
tac