What's the backstory on High-Spade?

I like this movie a lot, in part because the characters are interesting enough to want to know more about (outside of what happened in the film). I find myself wondering about Lin & High-Spade's experiences in the War Between the States -- that could have been the stuff of a prequel movie. And I really want to know what Lin did for High-Spade that made H-S feel so indebted to Lin that he would ride across (at least) Texas with him to find Dutch Henry Brown. Any thoughts?