You could put the numbers through the JC look up with every letter and see if you get any matching descriptions. Quarter horses usually have 4 or 5 numbers followed by a letter so you could also call up AQHA and ask.
Standardbred is 5 characters, but at least one of them (sometimes more) will be a letter. For a horse of that age, the first character would be a letter. Is it possible the first character in the tattoo is something that could be confused (B instead of 3, S instead of 5, etc)? If the horse is really, really old (over 30) the last character would be a letter, and probably one near the end of the alphabet.