I might suggest you read Dwayne Kling's Every Light Was On, Bill Harrah and His Clubs Remembered. This is an oral history with interviews by Mr. Kling, edited by R.T. King. You may order this book by using the link at the end of this post. The second chapter has a detailed rundown of the Harrah's slot operation and as most often the case, Harrah's procedures and practices became industry standard. A lot of info about the place slot held in casino operations, how new types of slots were developed and some surprises found when opening slot machines Harrah's purchased from Montana! You'll end up with a lot of insight to the era your machines were used. From the index:

JAMES CASELLI
Reno
Slot operations and slot machines. Thieves. Rome Andreotti's qualities. Golf tournaments. Premium points. Managment training. The "Jerrari." Harrah's procedures. Keno. Racial discrimination. Holiday Inns takes over. Slot cheaters-the old days.