We are a small, rural bank with no large commercial customers such as WalMart, Target, Publix. Our MSBs are very small commercial customers.

How in depth should the review of a small MSB's AML documentation be - for instance, Mom & Pop convenience stores? Should we actually review their AML written policy, procedures, training, etc? What if they don't have documentation for us to review? CTRs on one small MSB customer are filed at a maximum of 6 per year. Our other small MSB customers have never had a CTR filed in the three years I've been with this bank.

I would ensure they have it, but based on your information on the amount of cash they are dealing with, I would not be concerned about doing a review of it. And FinCEN has stated that we are not to be their overseers.
If they do not have an AML policy and procedures then I would direct them to where they can get a sample and ensure that they print them out and put them in place. As a NBFI they are required to have these.
I assume that you have verified with msb.gov that they are registered?
Hope this helps you.

Thanks for your response. I was thinking along similar lines, but wanted confirmation I wasn't just doing some wishful thinking.