I have SR (for Songcatcher Ranch) registered as a suffix with AMHR. I will only use it if the last word in my horse's name is registered as a suffix by another farm (never so far). Songcatcher is my registered prefix with both registries.
I see that is probably not the information you are looking for. In the AMHA Studbook Online, Horses with the SR prefix seem to be bred by Sid or Ruth Ann Hawkins or by Frances O Evans Seldom Rest Farm.