The 7 pin sidelock is for reference sake similar to the Holland and Holland bar action lock which is supposedly the most copied lock in the world. If you count all the pins including the tumbler and the mainspring pin, you come up with 9 pins. Most back action locks I have seen have less than 9, but some have that many pins. I think as a general rule, and barring the pinless lock, the less pins the less sophisticated the lock.....or more primitive the lock. For example, the L. C. Smith lock, which is a 4 pin lock (maybe less if you don't count the tumbler) is a back action lock that lacks intercepting safety sears and I think most would agree, more primitive than a Holland lock, yet probably just as functional. At least this is the way I look at your question which I have pondered myself.
