They look like some slugs from Latvia, I don't remeber the brand. They could also be a version Of the French Balle Blondeau. The French slug is supposed to be Steel, with lead alloy driving bands.
I have not loaded any 16 gauge slugs yet, but have loaded thousands of 12 gauge. What kind of velocities, and pressures are you looking for? What brands of powders do you have available? In the 15 gauge you will need to use slow powders to achieve velocities over 1300-1350FPS with that weight. In a modern gun, you could use IMR 4756, IMR 4759, Alliant Blue Dot, and Steel. Perhaps Hodgon;'s longshot would also work.
I would start with a plastic case, unless brass is available, then add the powder charge. Top it with a platic gas seal, and a hard filler wad, then a cork wad, and another hard filler under the slug.
Greg