The easiest way to tell if you have a Single Valve or Double Valve Standard Player Action is to look at the space directly below the Spoolbox. The Spoolbox is mounted on a Deck Board which is about 5/8" thick. Between the Deck Board and the Stack, there is a 3" space. In that space there will be a piece of wood. If that piece of wood appears to be held in place by 7-8 small screws, you have a Single Valve action. A double valve action will have two rows of screws that are spacing about 1-1/4" from each other that run the length of the upper section. Below is a picture of a Single Valve Standard Action.

I'll try to get some pictures to make this easier....

The first Lower Section pictured below has TWO Reservoirs. The one below that has only One.