Complex sentence: The Trinity Gadget was a Y-1561 device very similar to the Fat Man used a few weeks later in the bombing of Nagasaki, with only minor differences, the most obvious being the absence of the external ballistic casing.
Split into simpler sentences: The Trinity Gadget was officially a Y-1561 device, as was the Fat Man used a few weeks later in the bombing of Nagasaki. The two were very similar, with with only minor differences, the most obvious being the absence of the external ballistic casing.