What is the difference between the 471U, the 472U, and 473U strikes?

The 471U is for a 5547 with a metal door and frame.
The 472U is for a 5547WDC with a wood frame and a 1-3/4" thick door. 
The 473U is for a 5547WDC with a wood frame and a 2-1/4" thick door. 

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