The NVR3816 is a true 2U 19" rack mountable NVR.
The NVR3416R is a true 1U 19" rack mountable NVR.
NVR3416 & NVR3216
The NVR3416 & NVR3216 are not true 19" units. The NVR3416 & NVR3216 are supplied with mounting brackets to extend the NVR to allow it to be installed in a 19" rack. The NVR's are also slightly larger than 1U in height.
If an NVR3416 or NVR3216 are being installed in a 19" rack, there is a mounting fascia available (BTE07K) to make the NVR a true 2U 19" product.
The fascia part number is BTE07K for more information click here