The IT-E151 is for rack mounting of compatible products in either a side-by-side configuration, or s single unit configuration where a provided blank is used to fill the empty space. The dimensions for the kit are 5.24" (133mm) by 19" (482.6mm).
For the following models:
1785B, 1786B, 1787B, 1788
9120A, 9121A, 9122A, 9123A, 9124, 9130