johnnyatari,
Yes, that machine is an "M" Mills machine. As for the year, that is a bit tough to pinpoint. It could be as early as 1968. Sometimes you may find a date on a casting or two.
The pay glass. I have probably seen 3 of them pop up on eBay in the last 3-4 years. So, you may want to keep an eye out there. If you are in a hurry, you can contact Squires & Corrie, but it will be $$$.
I would have liked to see how the jackpot would have been installed and worked as they were original produced without them. The "Attendants" would normally pay the jackpots beyond the 20 coins.