"Thanks, I understand. I think the gun was purchased about 2003 so looks like he bought it used, but no way to determine it's true age without the $50 letter."

Max42, a letter will not get you the "true age," only the shipping date which is completely different from the manufacturing date. It's doubtful if you need to spend $50 on a factory letter in this case as the information it provides is unlikely to be particularly valuable. By the way, the M&P Model 10-6 production began in 1962. From your serial number, as previously stated, yours is likely to date from 1969-70 but could have shipped much later.