There are a couple of US military manuals I found that detail some care and feeding. The machine itself has few differences (other than the size) from a 15. Find one adjuster's manual for a 15 and you have the basics for both.
One main difference with the 31-15 is that it's timed by the needlebar only as far as I saw. The Adjuster's manual for the 15 says the timing is factory set but people say they time them. I've never had to. Those suckers take a beating and don't go out of time. I've had them show up with just a 1/4 of a needle left and the needleplates looking like they've been used for target practice.