Welcome to the forum. The gun you describe would be a Military & Police Third Model (Model of 1905), Fourth change. It dates to 1924, give or take a year. The fact that the stocks have no medallions is consistent with this date (the company did not use medallions during the 1920s), so they may be original to the gun. If you want to check, pull the stocks and see if the serial number is written on or pressed into the back of one panel.
I don't think you should feel any pain about the price you paid; sounds good to me.
Is there any chance the gun was refinished at some point? Guns with 95% original nickel are rare after 85 years, though there are some around. One possible tip off would be if the trigger and hammer are nickeled as well. These parts would show case hardening colors if original or if left untouched in a sensitive refinishing.
Can you post pictures of the gun? Lots of M&P fans here. Here's a thread on how to do it if you need some tips:
http://smith-wessonforum.com/smith-wesson-forum-office/78728-posting-photos-forum-new-format.html