In no particular order just as I am asked by customers, often there will be a range of choices available, if so to prevent confusion I will give the simplest answer I can, which will be in the form of my recommendation. So todays answers are
For a Horizon Stitchliner 5500 I recommend G5HZHD251/2
Hans Muller saddle stitcher, Is an early version of a Muller Martini. You need DB75HD251/2 ( for 0.6mm wire or smaller)
Laconda stitcher use 2601AOHD251/2 or G5HD251/2 depending on your preference, both heads suit this machine
If you are not sure then ask or send me some photo's.