We always put the PM in the last available slot, can be 1,2,3,or 4.

Reason being is so you don't have a hole in your extension numbering plan. It will work in any slot though I doubt that you will get it to fit in the center and the processor anywhere else without a hammer. Matter of fact it is a habit of mine to put any 200 and 400 mods after the messaging because there used to be an issue with older systems.

-Hal

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