"Memory voltage for new Haswell iMacs?"に関する回答The Haswell iMacs support both "normal" 1.5 V DDR3 and "low voltage" 1.35 V DDR3L SODIMMs. In order to operate at the lower voltage level (which consumes less power and generates less heat), you must use all DDR3L modules, otherwise they will operate at 1.5 V since the memory bus can only be driven at a single voltage.
"Type of Thunderbolt chip in 2013 iMac"に関する回答DSL3510L "Cactus Ridge" all around again this time. Andrew was kind enough to post links to some close-ups in the comments section of this blog post.